MFC r336262: Fix machdep_boot.c

A last minute change made this no longer compile. Pass the right arg
and eliminate now-unused variables from the code.

MFC r336283: Fix build after r344378

Eliminate an unused var warning-error; the var is used only when parsing
linux-style boot args, so wrap it in the appropriate ifdef.

MFC r336244, r336246-r336247: Standardize boot arg parsing

r336244:
Create helper functions for parsing boot args.

boot_parse_arg to parse a single arg
boot_parse_cmdline to parse a command line string
boot_parse_args to parse all the args in a vector
boot_howto_to_env Convert howto bits to env vars
boot_env_to_howto Return howto mask mased on what's set in the environment.

All these routines return an int that's the bitmask of the args
translated to RB_* flags. As a special case, the 'S' flag sets the
comconsole_speed env var. Any arg that looks like a=b will set the env
key 'a' to value 'b'. If =b is omitted, 'a' is set to '1'. This
should help us reduce the number of redundant copies of these routines
in the tree. It should also give a more uniform experience between
platforms.

Also, invent a new flag RB_PROBE that's set when 'P' is parsed. On
x86 + BIOS, this means 'probe for the keyboard, and if it's not there
set both RB_MULTIPLE and RB_SERIAL (which means show the output on
both video and serial consoles, but make serial primary). Others it
may be some similar concept of probing, but it's loader dependent
what, exactly, it means.

These routines are suitable for /boot/loader and/or the kernel,
though they may not be suitable for the tightly hand-rolled-for-space
environments like boot2.

r336246:
Eliminate boot loader copies of boot arg parsing.

Eliminate 4 of the copies of the arg parsing in /boot/laoder
by using boot_parse_cmdline.

r336247:
Transition to boot_env_to_howto and boot_howto_to_env in the boot
loader.

MFC r329989, r330044

r329989:
Add support for booting into kdb on arm platforms when the RB_KDB is set
(using "boot -d" at the loader propmt or setting boot_ddb in loader.conf).

Submitted by: Thomas Skibo <thomasskibo@yahoo.com>
Differential Revision: https://reviews.freebsd.org/D14428

r330044:
Add a hw.model sysctl oid for armv6/7 which reports the CPU model, similar
to what other arches (all except riscv and armv4/5) do.

Submitted by: Hyun Hwang <hyun@caffeinated.codes>
Differential Revision: https://reviews.freebsd.org/D14465

MFC r327367:

Make kernel option KERNVIRTADDR optional, remove it from std.<platform>
files that can use the default value.

It used to be required that the low-order bits of KERNVIRTADDR matched
the low-order bits of the physical load address for all arm platforms.
That hasn't been a requirement for armv6 platforms since FreeBSD 10.
There is no longer any relationship between load addr and KERNVIRTADDR
except that both must be aligned to a 2 MiB boundary.

This change makes the default KERNVIRTADDR value 0xc0000000, and removes the
options from all the platforms that can use the default value. The default
is now defined in vmparam.h, and that file is now included in a few new
places that reference KERNVIRTADDR, since it may not come in via the
forced-include of opt_global.h on the compile command line.

MFC r303261,r315059:

r303261:
Add more UEFI/e820 memory types from latest specifications.
r315059:
Split overbloated machep.c to multiple files and do basic cleanup of these
fragments.