Add a u-boot shell command mode.
Add a u-boot shell command mode.

Input "uboot" to enter u-boot shell command mode.
In u-boot shell command mode, input "exit" will exit the
mode and go back to FB shell mode.

Signed-off-by: Shenwei Wang <9d62adf49438fd6148b1e2de87b5beacc5976c46@nxp.com>

NXPmicro/mfgtools,NXPmicro/mfgtools,NXPmicro/mfgtools