Condition building PCBIOS legacy images on MACHINE_FEATURES containing "pcbios"
or not containing "efi". This ensures existing BSPs will continue to get the
old PCBIOS legacy-only images. New BSPs can add "efi", "pcbios", or both. The
images created likewise support one or the other or both.
EFI-only ISO images are untested.
Signed-off-by: Darren Hart <dvhart@linux.intel.com>