main/postmarketos-mkinitfs: rename usb_rndis_function variable
The variable describes the configfs function name to use, no matter if it's rndis, ncm, ecm or any other function. So change the name of the variable, the string we write to the configuration file and some comments.
See also #1797 (closed)
TODO After(?) merging: Update docs at https://wiki.postmarketos.org/wiki/Deviceinfo_reference#usb
Edited by Administrator