usbinstall.sh: clear data between 512B and 1MiB to fix grub-install error when old GPT table was present

src/usbinstall.sh

@@ -209,6 +209,8 @@ fi
 
 set_status $"Partitioning $SELECT_USB..."
 echo $"Partitioning $SELECT_USB..."
+# Wipe any old GPT table or grub-install may fail due to mmultiple partition labels
+sudo dd if=/dev/zero of=$SELECT_USB bs=512 count=2047 skip=1
 # Start from a new partition table
 sudo parted -s $SELECT_USB mktable msdos &>>$LOGFILE
 if [ "$?" != "0" ]; then