bootloader: fold all Android Bootloader specific logic into prepare-root

bootloader: fold all Android Bootloader specific logic into prepare-root

Now that we use androidboot.slot_suffix karg to determine whether we
boot into /ostree/root.a or /ostree/root.b, we can use ostree= karg
simply for parsing the stateroot, although we will still boot into
what's pointed to by /ostree/root.a or /ostree/root.b.