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.

src/shared/gpt.c

@@ -146,3 +146,14 @@ bool gpt_partition_type_is_usr_verity(sd_id128_t id) {
                 sd_id128_equal(id, GPT_USR_RISCV32_VERITY) ||
                 sd_id128_equal(id, GPT_USR_RISCV64_VERITY);
 }
+
+bool gpt_partition_type_knows_read_only(sd_id128_t id) {
+        return gpt_partition_type_is_root(id) ||
+                gpt_partition_type_is_usr(id) ||
+                sd_id128_equal(id, GPT_HOME) ||
+                sd_id128_equal(id, GPT_SRV) ||
+                sd_id128_equal(id, GPT_VAR) ||
+                sd_id128_equal(id, GPT_TMP) ||
+                gpt_partition_type_is_root_verity(id) || /* pretty much implied, but let's set the bit to make things really clear */
+                gpt_partition_type_is_usr_verity(id);    /* ditto */
+}

src/shared/gpt.h

@@ -133,3 +133,5 @@ bool gpt_partition_type_is_root(sd_id128_t id);
 bool gpt_partition_type_is_root_verity(sd_id128_t id);
 bool gpt_partition_type_is_usr(sd_id128_t id);
 bool gpt_partition_type_is_usr_verity(sd_id128_t id);
+
+bool gpt_partition_type_knows_read_only(sd_id128_t id);