- add usage information
- add option parsing:
  -v verbose
  -v -v more verbose
  -l list b´possible boot sources
  -d dryrun

The dryrun option sets the global variables necessary for booting
but does not actually boot the system. This way it is possible to
make additional adjustments to the boot variables and then invoke
bootm manually.

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