qemu: Add a bootmeth for qfw

This supports reading a kernel and ramdisk from qfw, then loading it with
either the booti or bootz commands.

For now this uses the existing booti and bootz commands, rather than
trying to call that functionality directly (e.g. do_bootm_states()). It
does not require the HUSH parser though, which helps a little with size.

Signed-off-by: Simon Glass <sjg@chromium.org>