stand: Enable Spleen 32x64 font in the bootloader

Spleen 32x64 provides my 14" 2560x1600 display with the ancestral 80x25,
BSD standard console. We already include it in contrib, and it is built
for the runtime vt(4) font directory, but was previously unavailable to
the bootloader. Enable it in the build, selectable in loader.conf with:

screen.font="32x64"

MFC after: 3 days
Reviewed by: adrian, carlavilla, emaste, imp, mhorne, tsoome
Approved by: imp, tsoome (loader)
Approved by: mhorne (mentor)
Differential Revision: https://reviews.freebsd.org/D49768