[coreboot] Coreboot crossgcc won't build without allow other toolchains enabled

Hi,

if i try to build the cross gcc (make crossgcc) make tells me to use the
coreboot tool chain.
This behavior seems to be a result of comit 9b0de . With "allow other
toolchains" enabled
i can build crossgcc. I assume the toolchain check needs to be disabled
for cross gcc or only enabled for coreboot itself and payloads.

cheers derpeter