u-boot: fsl_updater: handle uboot commands from utp messages
If receiving a message starting with "ubootcmd" string from utp,
the rest of the string will be interpreted as a uboot command and
try to be executed. This way all uboot command functionalities are
exposed to the mfg user.
Signed-off-by: Juan Gutierrez <b44802@freescale.com>