roms: build two seabios binaries

roms: build two seabios binaries

Adding xhci support to seabios made it jump over the 128k line.
Changing the bios size breaks migration, so we have to keep a
128k seabios binary for old machine types.  New machine types can
use a large 256k bios which should be big enougth for a while.

This patch updates the seabios build process to build seabios twice,
once full featured and once with xen and xhci turned off so the
resulting binary is small enougth to fit into 128k.

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