beckham: Create /data/gbfirmware for ModService

* Do this by creating init.sys.beckham.rc which will run from
  /system during post-fs-data. This way the directory gets created
  and proper sepolicies are set and ModService is satisfied.

* Update sepolicy accordingly

* Also, change gbfirmware_file -> greybus_fw_data_file which is the
  same as the stock label.

Change-Id: I8ee402e49dee8f95133634d57b48354a7b2acf43