buildroot

support/testing: add tests for ATF

These new tests only do build tests, but allow to quickly verify that
the ATF/U-Boot combinations for vexpress, Allwinner and Marvell
platforms all continue to build properly.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>