lantiq: introduce lantiq_is_vdsl_system

Move the code to check if the current system is a system with vdsl
support to a dedicate function to make it reusable.

Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/lantiq/base-files/lib/functions/lantiq.sh

@@ -39,3 +39,7 @@ lantiq_get_dt_led() {
 
 	echo "$label"
 }
+
+lantiq_is_vdsl_system() {
+	grep -qE "system type.*: (VR9|xRX200)" /proc/cpuinfo
+}