docs: hardware.rst: explain how to get the image name for a new device (#1335)

docs/dev/hardware.rst

@@ -31,6 +31,10 @@ can work::
 
     lua -e 'print(require("platform_info").get_image_name())'
 
+While porting Gluon to a new device, it might happen that the profile name is un-
+known. Best practise is to generate an image first by using an arbitrary value
+and then executing the lua command on the device and use its output from then on.
+
 The second parameter defines the name of the image files generated by LEDE. Usually,
 it is also the LEDE profile name; for devices that still use the old image build
 code, a third parameter with the LEDE profile name can be passed. The profile names