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>

Add support for handling this concept in bootflows. Update the 'bootflow'
command to allow only the normal bootmeths to be used. This alllows
skipping EFI bootmgr and VBE, for example.

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

With global bootmeths we want to scan without a bootdev. Update the logic
to allow this.

Change the bootflow command to show the bootdev only when valid.

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

Add support for bootflows, including maintaining a list of them and
iterating to find them.

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