GPT_BOTH_VALID is 4 bits so simple a boolean check is not sufficient.
This broken condition allowed gptprio to trust bogus disk locations in
headers that were marked invalid causing arbitrary disk corruption.
Basic usage would look something like this:
gptprio.next -d usr_dev -u usr_uuid
linuxefi ($usr_dev)/boot/vmlinuz mount.usr=PARTUUID=$usr_uuid
After booting the system should set the 'successful' bit on the
partition that was used.