Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

boot0: add a note about BIOS-supported serial rates

We plan to increase the default serial rate to 115200 (see review
D36295) but early boot components that use BIOS interfaces do not
support higher rates. Add a note to that effect.

Reported by: imp
Sponsored by: The FreeBSD Foundation

boot0: Drop the BOOT_BOOT0_ORG option.

This shouldn't be an option (and I added it in the first place back in
4ae4202e705cf6a12aa3cbb49efb153879a6bc2f and
83f4b92050cbf8913d5c0aac16726c783d8b2d9c). However, unlike the other
knobs I added back then, this really shouldn't be a knob since it is
hardcoded in the source.

boot0: Expand the description of BOOT_BOOT0_ORG.

This really shouldn't even be an option given it is hardcoded as a
constant named ORIGIN in the assembly. mbr.S also uses 0x600 and
hardcodes it in both the assembly and the Makefile.

Revert "stand: Remove double words in source code comments"

The sentence, "The base address that we the boot0 code to to run it."
is correct.

Reported by: jrtc27

This reverts commit b12ccd0bb1b37f32e972bb3e945e4025fe409e2f.

stand: Remove double words in source code comments

- s/to to/to/
- s/value value/value/

MFC after: 5 days

stand: Document that boot0 uses BIOS

And thus has a limited range of supported baud rates. Also add that
setting BOOT_BOOT0_COMCONSOLE_SPEED=0 will leave it unchanged which
sometimes can give you 115200 if the BIOS initialized things outside of
the normal BIOS baud rates (which many x86 enbedded-targetted boards
do).

Sponsored by: Netflix
Reviewed by: emaste, manu (earlier versions)
Suggestions by: jhb
Differential Revision: https://reviews.freebsd.org/D36300

Centralize several variables.

MK_CTF, MK_SSP, MK_PROFILE, NO_PIC, and INTERNALLIB are always the
same, so set them in defs.mk. MAN= is common, so set it here too.
This removes a lot of boring repetition from the Makefiles that added
almost no value.

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

Sponsored by: Netflix