fix: ship Pi images with MontaukOS UEFI boot entry

scripts/montauk-imager.py

@@ -26,6 +26,9 @@
 #                                 if it differs, and the UEFI firmware
 #                                 RPI_EFI.fd (which holds the firmware's own
 #                                 settings) is only replaced with --firmware.
+#                                 That also copies the image's MBR disk ID to
+#                                 the card: the new RPI_EFI.fd's MontaukOS
+#                                 boot entry finds the ESP by it.
 #           root (ext2, MONTAUKROOT)
 #                                 os/, apps/, sdk/, boot/ and any other system
 #                                 directory: new and changed files written.
@@ -644,6 +647,20 @@ def expand_root(rep, name):
 
 # ==== Flash ====
 
+def copy_disk_id(rep, name, raw):
+    """Give the card the image's MBR disk ID (the firmware's boot entry names the ESP by it)."""
+    with open(raw, "rb") as f:
+        disk_id = f.read(512)[440:444]
+    fd = os.open(f"/dev/{name}", os.O_RDWR | os.O_EXCL | os.O_CLOEXEC)
+    try:
+        if os.pread(fd, 4, 440) != disk_id:
+            os.pwrite(fd, disk_id, 440)
+            os.fsync(fd)
+            rep.log(f"MBR disk ID set to 0x{struct.unpack('<I', disk_id)[0]:08x}")
+    finally:
+        os.close(fd)
+
+
 def cmd_flash(rep, args):
     img = Image(args.image)
     name = check_target(rep, args.device)
@@ -1025,6 +1042,8 @@ def update(rep, args, img, name):
             if args.expand:
                 expand_root(rep, name)
                 esp, root, _ = card_layout(rep, name)
+        if args.firmware:
+            copy_disk_id(rep, name, raw)
 
         with contextlib.ExitStack() as stack:
             rep.stage("Mounting")