i386: Mark the obsolete fields in bootinfo with _was_

Mark the obsolete fields in bootinfo with _was_. Note that the fields
from bi_memdesc_version to the end of the structure never were used in a
release. They were added in April 2010 for i386 EFI booting. The boot
loader set these fields though 2019, but no kernel ever looked at them
and we never supported i386 EFI booting, and likely never will in this
form. They can likely be deleted entirely in the future, but locore.S
needs to change to do that (it also needs to change to drop support for
really old booting scenarios as well, which will eliminate bi_endcommon
too).

All the other fields haven't been used since the 4.x -> 5.x cutover of
the wdc driver to ata.

The bi_bios_dev field is used in the handoff between bootXX and the
loader. The loader uses it to determine what disk it was loaded off of
to detmerine the default root filesystem. It's not used by the i386
kernel anymore to determine anything.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D36544