loader: Fix boot menu on BIOS

Only the gfx-enabled boot loader supports unicode. Otherwise, we have to
use the old cons25 / ibmpc upper code page drawing characters. Check to
see if we have the gfx.term_drawbox function. If we do, we support the
unicode drawing characters. If we don't, then we have an older loader
that doesn't support it *OR* we have the reduced function, text-only
boot loader. In either of those cases, we need to use the old graphics
characters. Abstract all those details into core.hasUnicode function.

PR: 282465
MFC After: 2 day
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D47403