stand: Install gptboot.efi(8)

We need to include bsd.init.mk first when we have man pages in the boot
loader.

Sponsored by: Netflix

Remove $FreeBSD$: one-line sh pattern

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

gptboot.efi: Add man page

Add a man page for gptboot.efi. Describe when and how to use this as it differs
from the BIOS cases. Include cross reference for the preferred method described
in efibootmgr(8) as well as cross links in both gptboot(8) and gptboot.efi(8) to
the other.

This man page was heavily copied from the gptboot.8 man page by Warren Block.
They are different enough to need separate man pages for clarity, but there's
enough similarity that I worry about the duplication. In the really long term,
gptboot(8) will disappear, so having the same info here will help when that
day comes. In the short to medium term, the information is likely to not
change in gptboot(8) and any changes to gptboot.efi(8) will be easier to
make in a separate copy.

loader.efi(8) needs a complete rewrite from scratch, otherwise I'd have
referenced gptboot.efi(8) from there.

Suggetions from: cress@, mhorne@
Reviewed by: rpokala@
Differential Revision: https://reviews.freebsd.org/D29591

Only set WARNS if not defined

This would allow interested parties to do experimental runs with an
environment set appropriately to raise all the warnings throughout the
build; e.g. env WARNS=6 NO_WERROR=yes buildworld.

Not currently touching the numerous instances in ^/tools.

MFC after: 1 week

loader: remove libsa/crc32.c and use version from zlib

we have crc32(const void *, size_t) in libsa. Unfortunately zlib has
crc32(long, const unigned char *, unsigned) and we have conflict.

Since we do build libsa with zlib, we can use zlib version instead.

Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D24068

The efifat files are no longer used: remove the code to build them

Reviewed by: imp, tsoome, emaste
Differential Revision: https://reviews.freebsd.org/D20562

Create gptboot.efi

This is a primary boot loader that is intended to implement the
gptboot partition selection algorithm just like we did for BIOS
booting. While the preferred method for UEFI is to use the UEFI Boot
Manager protocol, there are situations where that can't be done: some
BIOS makers interfere with the protocol in unhelpful ways, there's a
new standard for a zero variable write from the client OS, and finally
for USB drives that might be mobile between systems with multiple
partitions there needs to be a media stable way to select.

Reviewed by: tsoome, bcran
Differential Revision: https://reviews.freebsd.org/D20547