MFC r325834,r325997,326502: Move sys/boot to stand/

This is effectively a direct commit to stable/11, due to differences between
stable/11 and head. Changes to DTS in sys/boot/fdt/dts were often
accompanied by kernel changes. Many of these were also risc-v updates that
likely had many more dependencies to MFC.

Because of this, sys/boot/fdt/dts remains as-is while everything else in
sys/boot relocates to stand/.

r325834: Move sys/boot to stand. Fix all references to new location

r325997: Remove empty directories.

r326502: Document the sys/boot -> stand move in hier.7 and the top-level README.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

Make comconsole options set before its activation to be remembered.

MFC after: 2 weeks
Sponsored by: iXsystems, Inc.

MFi386: revision 245848

Always update the hw.uart.console hint.

MFi386: r241300

i386 comconsole: don't loop forever if hardware doesn't respond

- clear capability flags when hw timeouts
- retire comc_started status variable and directly use c_flags to see
if comconsole is selected for use

MFi386: revision 229435

Add special loader environment variables 'comconsole_port' and
'comconsole_pcidev'.

MFC after: 2 weeks

MFi386: revision 1.11.

Remove ifdef PC98.

Don't repeat selected defines from ns16550.h or sioreg.h. Just
include ns16550.h. The missing installation of ns16550.h was fixed
long ago and the misplaced defines in sioreg.h were fixed recently.

Use __FBSDID().
Also some minor style cleanups.

Merged from the following changes.

sys/boot/i386/btx/btx/Makefile 1.9
sys/boot/i386/btx/btx/btx.s 1.23
sys/boot/i386/libi386/biosdisk.c 1.32 and 1.33
sys/boot/i386/libi386/biosmem.c 1.5
sys/boot/i386/libi386/comconsole.c 1.8
sys/boot/i386/libi386/gatea20.c 1.3
sys/boot/i386/libi386/time.c 1.4
sys/boot/i386/libi386/vidconsole.c 1.13
sys/boot/i386/loader/Makefile 1.48
sys/boot/i386/loader/main.c 1.22

$Id$ -> $FreeBSD$

PC98 version of new boot loader. Because boot2 has not yet ported,
files in boot2 directory are copies from legacy biosboot.

Submitted by: IMAI Takeshi <take-i@ceres.dti.ne.jp>