loader: Font module is EFI dependent

The font module is part of the gfx_fb support. Since we share this file
between EFI and kboot, we only want to pass the font data to the kernel
when we're booting from EFI, not kboot.

Sponsored by: Netflix

(cherry picked from commit 784f925e507f441206279a1e8776629fa7cdd14c)