gluon-core: add sysconfig_isset function

package/gluon-core/files/lib/gluon/functions/sysconfig.sh

@@ -5,6 +5,10 @@ sysconfig() {
 	cat "$SYSCONFIGDIR/$1" 2>/dev/null
 }
 
+sysconfig_isset() {
+	test -e "$SYSCONFIGDIR/$1"
+}
+
 sysconfig_set() {
 	echo -n "$2" > "$SYSCONFIGDIR/$1"
 }