Add cmd_process() to process commands in one place

We currently have the same code in hush.c and main.c. This brings the
code into one place.

As an added feature, if the command function returns CMD_RET_USAGE then
cmd_process() will print a usage message for the command before
returning the standard failure code of 1.

ARM code size increases about 32 bytes with this clean-up.

Signed-off-by: Simon Glass <sjg@chromium.org>