Handle u-boot.rom signing (U-Boot as x86 BIOS replacement) the same way
that u-boot.img signing is handled.
(From OE-Core rev: 94e3f427bbeb005d8443e9d822c3182f280df470)
Signed-off-by: George McCollister <george.mccollister@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>