Patch merged into coreboot/master: build system: mark sub-make invocations as parallelizable

the following patch was just integrated into master:
commit 78a5f22994da63bb02882aacb68cbbb70c2606ac
Author: Patrick Georgi 
Date:   Mon Jan 30 15:29:34 2017 +0100

build system: mark sub-make invocations as parallelizable

We rely on gnu make, so we can expect the jobserver to be around in
    parallel builds, too. Avoids some make warnings and slightly speeds up
    the build if those sub-makes are executed (eg for arm-trusted-firmware
    and vboot).

Change-Id: I0e6a77f2813f7453d53e88e0214ad8c1b8689042
    Signed-off-by: Patrick Georgi 
    Reviewed-on: https://review.coreboot.org/18263
    Tested-by: build bot (Jenkins)
    Reviewed-by: Paul Menzel 
    Reviewed-by: Martin Roth

See https://review.coreboot.org/18263 for details.

-gerrit