uboot-imx

buildman: refactor help message

"buildman [options]" is displayed by default.

Append the rest of help messages to parser.usage
instead of replacing it.

Besides, "-b <branch>" is not mandatory since commit fea5858e.
Drop it from the usage.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>