loader: Add new option WITH_LOADER_BIOS_TEXTONLY

This option will omit all the graphics support, the teken terminal
library, video mode support, etc and support a simple, basic, text-only
video console for the x86 BIOS boot loader. It uses the FreeBSD 12
version of vidconsole.c. It defaults to NO.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D43912