Add Dell PowerEdge r440 and r640 elevation images (#3814)

* Add images for dell r440 and r640

* Add front and rear image support to PowerEdge-R440

* Add front and rear image attributes to PowerEdge-R640

device-types/Dell/PowerEdge-R440.yml

@@ -7,6 +7,8 @@ is_full_depth: true
 weight: 17.64
 weight_unit: kg
 airflow: front-to-rear
+front_image: true
+rear_image: true
 console-ports:
   - name: Serial
     type: de-9

device-types/Dell/PowerEdge-R640.yaml

@@ -7,6 +7,8 @@ is_full_depth: true
 subdevice_role: parent
 weight: 29
 weight_unit: kg
+front_image: true
+rear_image: true
 console-ports:
   - name: Serial
     type: de-9