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>