buildroot

arm-trusted-firmware: allow to generate the BL31 image

Some platforms (e.g Allwinner ARM64) don't build a FIP image out of
ATF, but only a BL31 image, which is then encapsulated in U-Boot. This
commit adds an ATF option to build such an image.

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