MFC r309412,r316109,r316132:

r309412 (by imp):

dd is currently a bootstrap tool. It really doesn't have any business
being a bootstrap tool. However, for reproducible build output,
FreeBSD added dd status=none because it was otherwise difficult to
suppress the status information, but retain any errors that might
happen. There's no real reason that dd has to be a build tool, other
than we use status=none unconditional. Remove dd from a bootstrap tool
entirely by only using status=none when available. This may also help
efforts to build the system on non-FreeBSD hosts as well.

r316109:

Don't hardcode input files for stage 1/2 bootloaders; use .ALLSRC instead

This is a better pattern to follow when creating the bootloaders and doing
the relevant space checks to make sure that the sizes aren't exceeded (and
thus, copy-pasting is a bit less error prone).

r316132:

Parameterize out 7680 (15 * 512) as BOOT2SIZE, similar to sys/boot/i386/zfsboot/...

This is being done to make it easier to change in the future--this action might be
needed sooner rather than later because of gcc 6.3.0 bailing, stating that there
is negative free space left (deficit) in the boot2 bootloader.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

cdboot, pxeldr: make use of bootargs.h instead of redefining flag constants

Reviewed by: jhb
MFC after: 1 month

Define several extra macros in bsd.sys.mk and sys/conf/kern.pre.mk, to
get rid of testing explicitly for clang (using ${CC:T:Mclang}) in
individual Makefiles.

Instead, use the following extra macros, for use with clang:
- NO_WERROR.clang (disables -Werror)
- NO_WCAST_ALIGN.clang (disables -Wcast-align)
- NO_WFORMAT.clang (disables -Wformat and friends)
- CLANG_NO_IAS (disables integrated assembler)
- CLANG_OPT_SMALL (adds flags for extra small size optimizations)

As a side effect, this enables setting CC/CXX/CPP in src.conf instead of
make.conf! For clang, use the following:

CC=clang
CXX=clang++
CPP=clang-cpp

MFC after: 2 weeks

Upgrade our copy of llvm/clang to r126079, from upstream's trunk.

This contains many improvements, primarily better C++ support, an
integrated assembler for x86 and support for -pg.

Pass the -N flag to linked via -Wl.

Approved by: ed (mentor)

Removed now redundant CLEANFILES assignments.

Not read enough of my patch by: obrien ;)

Use a more compact syntax for passing the "binary" options to 'ld'.

Simplify the building of our i386 'binary' boot components by directly
producing them using 'ld' options rather than post-processing with 'objcopy'.

Idea by: Ryan Sommers <ryans@gamersimpact.com>

Get rid of unnecessary use of m4(1) by using cpp(1) instead.
(John tells me there were problems when trying this before,
but it appears to be safe these day.)

OK'ed by: jhb
Repocopied by: joe

- Factor out -nostdlib to an upper level Makefile.inc.

- Now that bsd.prog.mk deals with programs linked with -nostdlib
better, and has a notion of an "internal" program, use PROG
where possible. This has a good impact on the contents of
.depend files and causes programs to be linked with cc(1).

XXX: boot2 couldn't be converted as it's actually two programs.

Tested on: i386, amd64

First round of cleanups to sys/boot/ makefiles:

- do not use PROG for what's not a real C program,
- use sys.mk transformation rules where possible,
- only create the "machine" symlink on AMD64,
- removed MAINTAINER lines in individual makefiles,
- added the LIBSTAND defitinion to <bsd.libnames.mk>,
- somewhat better contents in .depend files.

Tested on: i386, amd64
Prodded by: bde

Inherit BINDIR from a parent Makefile.inc.

Add BOOT_PXELDR_ALWAYS_SERIAL option which forces serial console.

Initiate deorbit burn for the i386-only a.out related support. Moves are
under way to move the remnants of the a.out toolchain to ports. As the
comment in src/Makefile said, this stuff is deprecated and one should not
expect this to remain beyond 4.0-REL. It has already lasted WAY beyond
that.

Notable exceptions:
gcc - I have not touched the a.out generation stuff there.
ldd/ldconfig - still have some code to interface with a.out rtld.
old as/ld/etc - I have not removed these yet, pending their move to ports.
some includes - necessary for ldd/ldconfig for now.

Tested on: i386 (extensively), alpha

Try #2 at having /boot/loader default to ELF. Have pxeldr build its
own a.out version of loader.bin rather than depend on ../loader/loader
being a.out.

MAN[1-9] -> MAN.

Add support for probing the keyboard from pxeboot which will behave
exactly the same as passing -P to boot2.

Submitted by: jhb

Remove commented out NOMAN variable.

Grrr, fix a silly 'movl' -> 'movw' typo in both pxeldr and cdldr. Also,
remove unnecessary cruft from the Makefiles for both.

Add man page for pxeboot.

Reviewed By: asmodai

Mega i386 loader commit.
- Don't hard code 0x10000 as the entry point for the loader. Instead add
src/sys/boot/i386/Makefile.inc which defines a make variable with the
entry point for the loader. Move the loader's entry point up to
0x20000, which makes PXE happy.
- Don't try to use cpp to parse btxldr for the optional BTXLDR_VERBOSE,
instead use m4 to achieve this. Also, add a BTXLDR_VERBOSE knob in the
btxldr Makefile to turn this option on.
- Redo parts of cdldr's Makefile so that it now builds and installs cdboot
instead of having i386/loader/Makefile do that. Also, add in some more
variables to make the pxeldr Makefile almost identical and thus to ease
maintainability.
- Teach cdldr about the a.out format. Cdldr now parsers the a.out header
of the loader binary and relocates it based on that. The entry point of
the loader no longer has to be hardcoded into cdldr. Also, the boot
info table from mkisofs is no longer required to get a useful cdboot.
- Update the lsdev function for BIOS disks to parse other file systems
(such as DOS FAT) that we currently support. This is still buggy as
it assumes that a floppy with a DOS boot sector actually has a MBR and
parses it as such. I'll be fixing this in the future.
- The biggie: Add in support for booting off of PXE-enabled network
adapters. Currently, we use the TFTP API provided by the PXE BIOS.
Eventually we will switch to using the low-level NIC driver thus
allowing both TFTP and NFS to be used, but for now it's just TFTP.

Submitted by: ps, alfred
Testing by: Benno Rice <benno@netizen.com.au>