bootm: Create a function to run through the booti states

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

So far this is not used.

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