loader: Make MK_LOADER_BIOS_TEXTONLY work

Select between text-only and graphical frame buffer consoles for the
BIOS boot loader. Pull one or the other in with #ifdef in conf.c. Add
gfx_bios.c for the few routines that are needed for the BIOS support of
gfx. These are stubbed out for text-only mode. Move bi_load_vbe_data
here since it's only used for the graphical frame buffer.

Note: This setup also allows us to build multiple BIOS loaders if we
have to, some with text-only and some graphical. We don't do this today.
We may be forced to turn this on in the future if ZFS keeps growing.

The size savings is 41k, which helps a lot with some of our users that
want to enable more options in the BIOS boot loader than are normally
safe to do, and they don't need graphics.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D43917