ARM: rpi_b: load /uEnv.txt from MMC at startup

ARM: rpi_b: load /uEnv.txt from MMC at startup

The Pi has no flash to store an environment in the usual fashion. However,
the user may wish to customize the environment. We know that the SD card
must be present, since that's where the boot ROM has loaded U-Boot from.
So, load uEnv.txt from there early during boot. This allows the user to
e.g. customize boot_targets, in order to automatically select network
boot.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>