Most rpi defconfigs use dtb overlays, but not rpi0 / rpi2 - Making it harder
to use overlays on those boards as the genimage files have to be tweaked.
To fix this, create the rpi-firmware/overlays directory in the post-build
script if needed and unconditionally include it in the genimage files so
rpi0/rpi2 works consistently with the other variants.
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
