refactor(behaviors): Make label property optional

Changed all code (except for layer names) which used the label property
to use DEVICE_DT_NAME() instead, which uses the label if set or falls
back to the full node name. This matches how Zephyr determines the node
names used with device_get_binding() and allows us to start removing the
deprecated label property from things.

app/dts/bindings/behaviors/zmk,behavior-sensor-rotate-var.yaml

@@ -8,7 +8,8 @@ compatible: "zmk,behavior-sensor-rotate-var"
 properties:
   label:
     type: string
-    required: true
+    required: false
+    deprecated: true
   "#sensor-binding-cells":
     type: int
     required: true