The variants of arm-trusted-firmware-mediatek and uboot-mediatek only
write to their own build directories and to files in the staging and
bin directories that have the variant in their name.
For the OpenWrt One, a clean build of the 9 TF-A variants takes 2 s
instead of 11 s, and of the 2 U-Boot variants 12 s instead of 21 s.
The images do not change.
Signed-off-by: John Crispin <john@phrozen.org>