gpt-auto-generator: optionally, set up root fs via dissection logic

gpt-auto-generator: optionally, set up root fs via dissection logic

This introduces root=dissect, which is a lot like root=gpt-auto, but
uses the image dissection logic to find the root partition. It's also
based on the GPT efi var telling us about the root disk, but instead of
just picking some suitable root disk manually it uses the dissection
logic.

This is a big step forward, since it means we can actually make
use of the image policy logic for the root fs too.