repart: add new ReadOnly= and Flags= settings for repart dropins

Let's make the GPT partition flags configurable when creating new
partitions. This is primarily useful for the read-only flag (which we
want to set for verity enabled partitions).

This adds two settings for this: Flags= and ReadOnly=, which strictly
speaking are redundant. The main reason to have both is that usually the
ReadOnly= setting is the one wants to control, and it' more generic.
Moreover we might later on introduce inherting of flags from CopyBlocks=
partitions, where one might want to control most flags as is except for
the RO flag and similar, hence let's keep them separate.