scripts: wipe gpt partition info after writing boot blocks

in case boot blocks from other dists like armbian or minimyth2 are used
it is a good idea to wipe the gpt partition info to avoid old remaining
gpt partition info from those boot blocks to confuse newly created mbr
partition based setups

scripts/create-image.sh

@@ -166,6 +166,10 @@ if [ "${CROSPARTS}" = "true" ]; then
 else
   if [ -f ${DOWNLOAD_DIR}/boot-${1}-${2}.dd ]; then
     dd if=${DOWNLOAD_DIR}/boot-${1}-${2}.dd of=/dev/loop0
+    # clear the partition table and reread it via partprobe in case the boot
+    # blocks provided contained a gpt partition table which might confuse later
+    sgdisk -Z /dev/loop0
+    partprobe /dev/loop0
   fi
 fi