fastboot: Add OEM run command

This adds the UUU UCmd functionality as an OEM command. While the
fastboot tool allows sending arbitrary commands as long as they are
prefixed with "oem". This allows running generic U-Boot commands over
fastboot without UUU, which is especially useful when not using USB.
This is really the route we should have gone in the first place when
adding these commands.

While we're here, clean up the UUU Kconfig a bit.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Heiko Schocher <hs@denx.de>