target/generic: add uartlite ttyULx devices to securetty

target/generic/target_busybox_skeleton/etc/securetty

@@ -10,3 +10,7 @@ ttyS0
 ttyS1
 ttyS2
 ttyS3
+ttyUL0
+ttyUL1
+ttyUL2
+ttyUL3

target/generic/target_skeleton/etc/securetty

@@ -10,3 +10,7 @@ ttyS0
 ttyS1
 ttyS2
 ttyS3
+ttyUL0
+ttyUL1
+ttyUL2
+ttyUL3