bootstd: Adjust the default bootmeth order

The existing distro scripts check extlinux and scripts before EFI. Adjust
the default ordering to do the same, to avoid breaking existing flows.

Add some documentation, mentioning that this order will likely change in
future.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Da Xue <da@libre.computer>