[coreboot] coreboot image signature

[Łukasz Dobrowolski]

Bear in mind that on some mainboards (at least the one that I'm trying
to port) the beginning of the rom is occupied by EC firmware blob. On
x86 you could check the reset vector at 0xFFFFFFF0 but i don't know if
it's the same way with all architectures. Also You would have to
somehow tell the difference between vendor bios and coreboot rom.

To me it seems that storing cb roms with ".cbrom" extension would be
more practical.