build/roms: Make coreboot crossgcc usable for payloads and modules

Add the coreboot-built cross-architecture toolchains to the PATH so that
modules and payloads can use them. When building for a foreign-arch
board, also export CROSS_COMPILE pointing to the appropriate prefix.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>