ar71xx-generic: make ubiquiti-rocket-m a separate device

Make ubiquiti-rocket-m a separate device rather than a bullet-m alias to
enable USB support and allow adding devive-specific packages.

targets/ar71xx-generic

@@ -221,12 +221,14 @@ device ubiquiti-airrouter ubnt-airrouter
 device ubiquiti-bullet-m ubnt-bullet-m
 alias ubiquiti-nanostation-loco-m2
 alias ubiquiti-nanostation-loco-m5
-alias ubiquiti-rocket-m2
-alias ubiquiti-rocket-m5
 alias ubiquiti-bullet-m2
 alias ubiquiti-bullet-m5
 alias ubiquiti-picostation-m2
 
+device ubiquiti-rocket-m ubnt-rocket-m
+alias ubiquiti-rocket-m2
+alias ubiquiti-rocket-m5
+
 device ubiquiti-nanostation-m ubnt-nano-m
 alias ubiquiti-nanostation-m2
 alias ubiquiti-nanostation-m5