bootm: Create a function to run through the bootz states

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

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