loader.efi: reworked framebuffer setup

Pass gfx_state to efi_find_framebuffer(), so we can pick between
GOP and UGA in efi_find_framebuffer(), also we can then
set up struct gen_fb in gfx_state from efifb and isolate efi fb data
processing into framebuffer.c.

This change does allow us to clean up efi_cons_init() and reduce
BS->LocateProtocol() calls.

A little downside is that we now need to translate gen_fb back to
efifb in bootinfo.c (for passing to kernel), and we need to add few
-I options to CFLAGS.