boot: Run the EFI bootmgr just before network devices

boot: Run the EFI bootmgr just before network devices

At present the EFI bootmgr scans all devices in the system before
deciding which one to boot. Ideally it would use the bootstd iterator
for this, but in the meantime, give it a lower priority, so it runs
just before the network devices.

Note that if there are no hunted network devices hunted, then it will
run at the end, after all bootdevs are exhausted. In other words, it
will always run.

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