bootstd: Allow scanning for global bootmeths separately

Typically we want to find and use global bootmeths first, since they have
the best idea of how the system should boot. We then use normal bootmeths
as a fallback.

Add the logic for this, putting global bootmeths at the end of the
ordering. We can then easily scan the global bootmeths first, then drop
them from the list for subsequent bootdev-centric scans.

This changes the ordering of global bootmeths, so update the
bootflow_system() accordingly.

Drop the comment from bootmeth_setup_iter_order() since this is an
exported function and it should be in the header file.

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