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>

The current way of handling things like EFI bootmgr is a bit odd, since
that bootmeth handles selection of the bootdev itself. VBE needs to work
the same way, so we should support it properly.

Add a flag that indicates that the bootmeth is global, rather than being
invoked on each bootdev. Provide a helper to read a bootflow from the
bootmeth.

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

Some bootmeths can provide information about what is available to boot.
For example, VBE simple provides access to the firmware state.

Add a new method for this, along with a sandbox test.

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

A bootmeth is a method of locating an operating system. For now, just
add the uclass itself. Drivers for particular bootmeths are added later.

If no bootmeths devices are included in the devicetree, create them
automatically. This avoids the need for boilerplate in the devicetree
files.

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