boot-directdisk: Allow for EFI-only boot direct disk images

boot-directdisk: Allow for EFI-only boot direct disk images

    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.

(From OE-Core rev: c58aceee7dc243467dd87f07ccc61859f8d945e6)

Signed-off-by: João Henrique Ferreira de Freitas <joaohf@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>