bootstd: Correct logic for single uclass

The current logic for "bootflow mmc" is flawed since it checks the
uclass of the bootdev instead of its parent, the media device. Correct
this and add a test that covers this scenario.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Ivan T.Ivanov <iivanov@suse.de>