refactor: refactor generate DISKID

As discussed in #71 changed behavior to
mentioned code by pelwel.

See #71 (comment)
for details.

Signed-off-by: Stephan Wendel <me@stephanwe.de>

usr/lib/raspberrypi-sys-mods/firstboot

@@ -63,11 +63,7 @@ get_variables () {
 fix_partuuid() {
   mount -o remount,rw "$ROOT_PART_DEV"
   mount -o remount,rw "$BOOT_PART_DEV"
-  MAJOR="$(uname -r | cut -f1 -d.)"
-  if [[ "$MAJOR" -eq "6" ]] && [[ -c /dev/hwrng ]]; then
-    dd if=/dev/hwrng of=/dev/urandom count=1 bs=256 status=none
-  fi
-  DISKID="$(tr -dc 'a-f0-9' < /dev/urandom | dd bs=1 count=8 2>/dev/null)"
+  DISKID="$(dd if=/dev/hwrng bs=4 count=1 | od -An -tx4 | cut -c2-9)"
   fdisk "$ROOT_DEV" > /dev/null <<EOF
 x
 i