USE_XARGS mode builds n boards in parallel (with 1 CPU each) instead of
building 1 board with n CPUs.
This requires the main build system to work under such circumstances.
Change-Id: Ib4571a78dfe78fd61ae5b26c18be9745bd8b3d52
Reviewed-on: http://review.coreboot.org/4485
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
589555109c