change device identifier to match machine identifier

this matches the output of
lua -e 'print(require(platform_info).get_image_name())'

This way the manifest file is accepted by the autoupdater

targets/ar71xx-generic

@@ -79,7 +79,7 @@ factory
 device gl-ar300m gl-ar300m
 factory
 
-device gl-ar750 gl-ar750
+device gl.inet-gl-ar750 gl-ar750
 packages $ATH10K_PACKAGES
 factory