Add a special command to load and start a bzImage on x86 ...

Other architectures are supporting the uImage format used by barebox's 'bootm'
command. x86 does'nt. So, we need a special command to be able to boot the
x86 specific bzImage format.

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