boot: Run global bootmeths after all bootdevs are exhausted

boot: Run global bootmeths after all bootdevs are exhausted

When there are no more bootdevs we should still go through the global
bootmeths, since some may not have yet been used, if their priority has
not yet come up.

Add a final check for this at the end of the iterator.

Update the documentation to match the new behaviour of global bootmeths.

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