bootm: Create a new boot_run() function to handle booting

Create a common function used by the three existing bootz/i/m_run()
functions, to reduce duplicated code.

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