arm: baltos: enable booting from USB

First of all U-Boot would search for a USB mass storage device
with either uEnv.txt or kernel-fit.itb and boot.

If USB mass storage device is not available or doesn't provide
these files then MMC will be tried.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>