Support generating GPT partition tables (MR 2160)

The PPP will, in combination with tow-boot, boot with GPT on the
internal storage. We need to support that on postmarketOS and as a first
step, let's make it possible to generate GPT images and boot from that

pmb/config/__init__.py

@@ -707,6 +707,7 @@ deviceinfo_attributes = [
     "sd_embed_firmware_step_size",
     "partition_blacklist",
     "boot_part_start",
+    "partition_type",
     "root_filesystem",
     "flash_kernel_on_update",
 

pmb/install/partition.py

@@ -71,8 +71,10 @@ def partition(args, size_boot, size_reserve):
     # will stop there (see #463).
     boot_part_start = args.deviceinfo["boot_part_start"] or "2048"
 
+    partition_type = args.deviceinfo["partition_type"] or "msdos"
+
     commands = [
-        ["mktable", "msdos"],
+        ["mktable", partition_type],
         ["mkpart", "primary", filesystem, boot_part_start + 's', mb_boot],
     ]