arm-bsp/boot-firmware: fix build race

As boot-firmware is heavily based on SCP-firmware it has the same build
race.  Luckily, the patch in SCP-firmware[1] applies cleanly to
boot-firmware.

[1] https://github.com/ARM-software/SCP-firmware/commit/34e1c04534607f5605255f39fb46e26261fc9c4e


Signed-off-by: Ross Burton <ross.burton@arm.com>

Change-Id: Iaa2f1bb71fce66589db530ea378d4ce995a6bed9
Signed-off-by: Jon Mason <jon.mason@arm.com>