Without arguments the 'md' command defaults to show address 0 which
likely results in a NULL pointer exception, so only three keystrokes
are necessary to crash barebox. Show usage instead if 'md' is invoked
without arguments, so that it at least requires an address to be given
to crash barebox. This increases the stability of barebox by 66%. Hurray!

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>