fff-gateway: Add sanity checks

This adds two checks:
- Does gateway config exist?
- Does gateway config version match?

Signed-off-by: Fabian Bläse <fabian@blaese.de>
Tested-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>

src/packages/fff/fff-gateway/files/etc/gateway.d/01-version

@@ -0,0 +1,19 @@
+configure() {
+	local expected_version=1
+	local config_version=$(uci -q get gateway.meta.config_version)
+
+	# check if gateway config exists
+	if ! uci show gateway > /dev/null; then
+		echo "ERROR: Gateway config could not be parsed or does not exist."
+
+		exit 1
+	fi
+
+	# check version of configuration
+	if [ "$config_version" != "$expected_version" ]; then
+		echo "ERROR: Invalid config version. Expected '$expected_version', got '$config_version'."
+		echo "Please check what has been changed and adjust your config appropriately."
+
+		exit 1
+	fi
+}