bootstd: Make bootdev_next_prio() continue after failure

bootstd: Make bootdev_next_prio() continue after failure

When a device fails to probe, the next device should be tried, until
either we find a suitable device or run out of devices. A device
should never be tried twice.

When we run out of devices of a particular priority, the hunter should
be used to generate devices of the next priority. Only if all attempts
fail should this function return an error.

Update the function to use the latent 'found' boolean to determine
whether another loop iteration is warranted, rather than setting 'dev'
to NULL, which creates confusion, suggesting that no devices have been
scanned and the whole process is starting from the beginning.

Note that the upcoming bootflow_efi() test is used to test this
behaviour.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: https://source.denx.de/u-boot/custodians/u-boot-dm/-/issues/17