bootm: Fix help message's sub-command ordering

The help message for the 'bootm' command listed the 'cmdline' and 'bdt'
sub-commands in the wrong order which resulted in the error below when
following the 'help' command's instructions:

  "Trying to execute a command out of order"

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>