bootm: Avoid returning error codes from command

bootm: Avoid returning error codes from command

Functions which implement commands must return a CMD_RET_... error code.
At present bootm can return a negative errno value in some cases, thus
causing strange behaviour such as trying to exit the shell and printing
usage information.

Fix this by returning the correct value.

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