postmarketos-initramfs: refactor find_root/boot_partition

These functions are almost identical and follow the same logic.
Introduce a generic find_partition function which they can both call
into so we can abstract the logic away from the specific usage and make
everything much cleaner.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>
Part-of: postmarketOS/pmaports!6172