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.

MFC r308089: zfsbootcfg: a simple tool to set next boot (one time)
options for zfsboot

There is a branch-specific change in sbin/zfsbootcfg/Makefile because of
LIBADD vs LDADD/DPADD.

MFC r280980:

Ensure the cross assembler, linker and objcopy are used for the build32
stage, just like for the regular world stage.

Reviewed by: rodrigc, imp, bapt, emaste
Differential Revision: https://reviews.freebsd.org/D2187

MFC r271609: add gptzfsboot.8, zfsboot.8 and zfsloader.8 manual pages

r261567: Build a 32-bit libstand under sys/boot/

A 32-bit libstand is needed on 64-bit platforms for use by various
bootloaders. Previously only the 32-bit version was built, installed
as /usr/lib/libstand.a.

A new 64-bit libstand consumer will arrive in the near future, so move
the bootloader-specific 32-bit version to sys/boot/libstand32/.

Explicitly link against this version in the 32-bit loaders.

r261614: Build a 32-bit libstand under sys/boot/ for ppc64

This change is equivalent to r261567 for i386/amd64.

Relnotes: Yes
Sponsored by: The FreeBSD Foundation

MFC r260334:

Split the last gcc-specific flags off into CFLAGS.gcc. This also
removes the need to use -Qunused-arguments for clang throughout the
tree.

MFC r260369:

Apply band-aid for 32-bit compat libs failures after r260334: put back
-Qunused-arguments for clang for now, until I can figure out a way to
make it unneeded in all scenarios. Sorry about the breakage.

MFC r260095:

For sys/boot/i386 and sys/boot/pc98, separate flags to be passed
directly to the linker (LD_FLAGS) from flags passed indirectly, via the
compiler driver (LDFLAGS).

This is because several Makefiles under sys/boot/i386 and sys/boot/pc98
use ${LD} directly to link, and the normal LDFLAGS value should not be
used in these cases.

MFC r280980:

Ensure the cross assembler, linker and objcopy are used for the build32
stage, just like for the regular world stage.

Reviewed by: rodrigc, imp, bapt, emaste
Differential Revision: https://reviews.freebsd.org/D2187

MFC r271609: add gptzfsboot.8, zfsboot.8 and zfsloader.8 manual pages

r261567: Build a 32-bit libstand under sys/boot/

A 32-bit libstand is needed on 64-bit platforms for use by various
bootloaders. Previously only the 32-bit version was built, installed
as /usr/lib/libstand.a.

A new 64-bit libstand consumer will arrive in the near future, so move
the bootloader-specific 32-bit version to sys/boot/libstand32/.

Explicitly link against this version in the 32-bit loaders.

r261614: Build a 32-bit libstand under sys/boot/ for ppc64

This change is equivalent to r261567 for i386/amd64.

Relnotes: Yes
Sponsored by: The FreeBSD Foundation

MFC r260334:

Split the last gcc-specific flags off into CFLAGS.gcc. This also
removes the need to use -Qunused-arguments for clang throughout the
tree.

MFC r260369:

Apply band-aid for 32-bit compat libs failures after r260334: put back
-Qunused-arguments for clang for now, until I can figure out a way to
make it unneeded in all scenarios. Sorry about the breakage.

MFC r260095:

For sys/boot/i386 and sys/boot/pc98, separate flags to be passed
directly to the linker (LD_FLAGS) from flags passed indirectly, via the
compiler driver (LDFLAGS).

This is because several Makefiles under sys/boot/i386 and sys/boot/pc98
use ${LD} directly to link, and the normal LDFLAGS value should not be
used in these cases.