fix: ship Pi images with MontaukOS UEFI boot entry

scripts/mk-rpi4-sd.sh

@@ -33,6 +33,13 @@
 # the image with e.g.
 #   sudo dd if=montauk-rpi4.img of=/dev/sdX bs=4M conv=fsync
 #
+# Boot order: the firmware's RPI_EFI.fd ships with an empty variable store,
+# which scripts/rpi4-efi-bootorder.py seeds with a "MontaukOS" Boot0000 first
+# in BootOrder, naming the ESP by this image's (random) MBR disk ID and
+# partition number. The card then boots without a trip to the boot menu,
+# ahead of the PXE/HTTP entries the firmware adds on its own, from the SD
+# slot or a USB drive alike.
+#
 # Firmware settings (UEFI setup, Esc at boot) MontaukOS relies on are the
 # defaults: System Table Mode "ACPI" and "Limit RAM to 3 GB" enabled (the
 # BCM2711's PCIe can only DMA below 3 GB, see the _DMA objects in its ACPI
@@ -150,13 +157,19 @@ if [ "$ROOTFS" = ext2 ]; then
         -d "$WORK/root" "$WORK/root.ext2" "${root_mib}M"
 fi
 
+# The MBR disk ID the firmware's boot entry names the ESP by.
+DISK_ID=0
+while [ $((DISK_ID)) -eq 0 ]; do
+    DISK_ID="0x$(od -An -N4 -tx4 /dev/urandom | tr -d ' ')"
+done
+
 rm -f "$OUTPUT"
 dd if=/dev/zero of="$OUTPUT" bs=1M count=0 seek=$((1 + esp_mib + root_mib)) status=none
 if [ "$ROOTFS" = ext2 ]; then
-    printf 'label: dos\nstart=2048, size=%dMiB, type=ef\nstart=%dMiB, type=83\n' \
-        "$esp_mib" "$((1 + esp_mib))"
+    printf 'label: dos\nlabel-id: %s\nstart=2048, size=%dMiB, type=ef\nstart=%dMiB, type=83\n' \
+        "$DISK_ID" "$esp_mib" "$((1 + esp_mib))"
 else
-    printf 'label: dos\nstart=2048, type=ef\n'
+    printf 'label: dos\nlabel-id: %s\nstart=2048, type=ef\n' "$DISK_ID"
 fi | PATH=$PATH:/usr/sbin:/sbin sfdisk --quiet "$OUTPUT"
 
 # The ESP is built as its own file: mtools sizes a filesystem at an @@offset
@@ -174,6 +187,10 @@ mformat -i "$IMG" -F -v MONTAUK ::
 for f in "$FW_DIR"/*; do
     case "$(basename "$f")" in
         Readme.md) ;;
+        RPI_EFI.fd)
+            python3 -I "$ROOT/scripts/rpi4-efi-bootorder.py" "$f" "$WORK/RPI_EFI.fd" \
+                "$DISK_ID" 1 2048 $((esp_mib * 2048))
+            mcopy -i "$IMG" "$WORK/RPI_EFI.fd" :: ;;
         config.txt)
             { cat "$f"; printf '\ncore_freq=500\ncore_freq_min=500\n'; } > "$WORK/config.txt"
             mcopy -i "$IMG" "$WORK/config.txt" :: ;;