bootstd: Detect empty bootmeth ordering

If the ordering produces no entries, this is an error. Report it, so that
the caller doesn't try to continue with a NULL bootmeth.

This fixes a crash in the bootflow_iter test when running with the sandbox
'default' device tree, instead of the required 'test' one.

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