disk-image-uefi.binary: shift down boot number to 13

Support some systems which don't handle partition numbers
higher than 15. (LP: #2072929)
Partition 16 was added for /boot to enable cloud FDE (commit a8b2a9b01)

live-build/functions

@@ -237,10 +237,13 @@ mount_disk_image() {
     mount_image ${disk_image} "${rootpart}"
     mount_partition "${rootfs_dev_mapper}" $mountpoint
 
-    local boot_dev="${loop_device}p16"
+    local boot_dev="${loop_device}p13"
     if flock -x ${loop_device} \
         [ -b ${boot_dev} -a -e $mountpoint/boot ]; then
-        flock -x ${loop_device} mount "${boot_dev}" $mountpoint/boot
+        # Only mount if assumed boot_dev is XBOOTLDR type
+        if sgdisk -i13 "${loop_device}" | grep -i BC13C2FF-59E6-4262-A352-B275FD6F7172 ; then
+            flock -x ${loop_device} mount "${boot_dev}" $mountpoint/boot
+        fi
     fi
 
     # Having one partition mounted should avoid udev-triggered partition

live-build/ubuntu-cpc/hooks.d/base/disk-image-uefi.binary

@@ -37,28 +37,28 @@ create_partitions() {
     case $ARCH in
         arm64|armhf)
             sgdisk "${disk_image}" \
+                   --new=13::1G \
+                   --typecode=13:ea00 \
                    --new=15:0:204800 \
                    --typecode=15:ef00 \
-                   --new=16::1G \
-                   --typecode=16:ea00 \
                    --new=1:
             ;;
         riscv64)
             sgdisk "${disk_image}" \
                    --set-alignment=2 \
+                   --new=13::1G \
+                   --typecode=13:ea00 \
                    --new=15::+106M \
                    --typecode=15:ef00 \
-                   --new=16::1G \
-                   --typecode=16:ea00 \
                    --new=1:: \
                    --attributes=1:set:2
             ;;
         amd64)
             sgdisk "${disk_image}" \
+                   --new=13::1G \
+                   --typecode=13:ea00 \
                    --new=14::+4M \
                    --new=15::+106M \
-                   --new=16::1G \
-                   --typecode=16:ea00 \
                    --new=1::
             sgdisk "${disk_image}" \
                    -t 14:ef02 \
@@ -71,7 +71,7 @@ create_partitions() {
 
 create_and_mount_boot_partitions() {
     uefi_dev="${loop_device}p15"
-    boot_dev="${loop_device}p16"
+    boot_dev="${loop_device}p13"
     mountpoint="$1"
 
     mkfs.vfat -F 32 -n UEFI "${uefi_dev}"