Prevent multiple vc4-kms-v3d overlay entries (#483)

Author: isius

board/raspberrypi/post-image.sh

@@ -103,12 +103,14 @@ __EOF__
 		;;
 		--add-vc4-kms-v3d-overlay)
 		# Enable VC4 overlay
-		echo "Adding 'dtoverlay=vc4-kms-v3d' to config.txt."
-		cat << __EOF__ >> "${BINARIES_DIR}/rpi-firmware/config.txt"
+		if ! grep -qE '^dtoverlay=vc4-kms-v3d' "${BINARIES_DIR}/rpi-firmware/config.txt"; then
+			echo "Adding 'dtoverlay=vc4-kms-v3d' to config.txt."
+			cat << __EOF__ >> "${BINARIES_DIR}/rpi-firmware/config.txt"
 
 # Add VC4 GPU support
 dtoverlay=vc4-kms-v3d-pi4
 __EOF__
+		fi
 		;;
                 --silent)
                 if ! grep -qE '^disable_splash=1' "${BINARIES_DIR}/rpi-firmware/config.txt"; then