bootm: Create a function to run through the bootm states

In quite a few places, the bootm command is used to handle a boot. We
want these to be done without needing CONFIG_CMDLINE, so add a new
bootm_run() function to handle this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>