arm-trusted-firmware: add BR2_TARGET_ARM_TRUSTED_FIRMWARE_UBOOT_AS_BL33

arm-trusted-firmware: add BR2_TARGET_ARM_TRUSTED_FIRMWARE_UBOOT_AS_BL33

ATF can be used in different ways:

 - ATF encapsulates U-Boot as the BL33, which is what is done on ARM
   Juno (currently supported in Buildroot) and Marvell platforms (soon
   to be supported)

 - U-Boot encapsulates ATF's BL31, which is what is done on Allwinner
   ARM64 platforms.

Until now we were assumming the former was always the case, but
obviously it isn't. Therefore, this patch adds an option that allows
to explicitly tell ATF that it encapsulates U-Boot as its BL33.

We adapt the only defconfig that uses ATF so that it enables this
option as appropriate.

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