ARM: rpi: support an environment

Enable ENV_IS_IN_FAT so that the environment can be stored persistently.
It's stored in the FAT partition that the RPi firmware requires. On most
RPis, this is on the SD card (which must be present in order for the
system to boot). On the CM this is on the built-in eMMC device.

Since we now have a persistent environment, there's no need to load
uEnv.txt at boot; we only did that to work around the lack of persistent
environment.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>