common, menu: show menu on startup if CONFIG_MENU_SHOW is defined

show a menu on startup instead running the shell.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Jason Hobbs <jason.hobbs@calxeda.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Acked-by: Jason Hobbs <jason.hobbs@calxeda.com>