gui-init: add Power Off to the Main Boot Menu

Add a main boot menu entry to power off. This enables users to
only verify the firmware integrity using OTP, and do nothing more.

After having left the device out of sight, one might want to do
a quick sanity check only.

Since we already have a script to safely power down, we make use of
it now.

initrd/bin/gui-init

@@ -175,6 +175,7 @@ while true; do
       --menu "$date\nTOTP: $TOTP | HOTP: $HOTP" 20 90 10 \
       'y' ' Default boot' \
       'a' ' Advanced Settings -->' \
+      'P' ' Power Off' \
       'x' ' Exit to recovery shell' \
       2>/tmp/whiptail || recovery "GUI menu failed"
 
@@ -330,6 +331,10 @@ while true; do
     continue
   fi
 
+  if [ "$totp_confirm" = "P" ]; then
+    poweroff
+  fi
+
   if [ "$totp_confirm" = "y" -o -n "$totp_confirm" ]; then
     # Try to boot the default
     mount_boot