boot: Support rescanning the global bootmeths

boot: Support rescanning the global bootmeths

Add the logic to scan through the global bootmeths for every new
bootdev, in preparation for allowing global bootmeths to select where in
the hunter ordering they go.

Use a new bootmeth_glob_allowed() function to check if a bootmeth is
allowed, ensuring that each can run at most once.

For now this has no actual effect, since the global bootmeths are
unconditionally processed at the start, with iter->methods_done being
updated to include all of them. Therefore when scanning again, no
unprocessed global bootmeths will be found.

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