Add vgabios build rules to roms/Makefile

This patch adds some glue to roms/Makefile to build vgabios binaries for
qemu.  It covers both the lgpl'ed vgabios implementation used by qemu
traditionally and the new seabios implementation.

The purpose of this patch is to (a) document the vgabios build process
and (b) simplify seavgabios testing for those who want to play with it.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>