Bug 935059 - Add partial FOTA support

Add --fota-type and --fota-files arguments to script to handle the
generation of a limited subset of the FOTA. With fota_type 'full', code
path should remain the same. But with 'partial', we will:
 - not format the partition
 - purge the directory specified in --fota-dirs
 - only get the files specified in the file specified as --fota-files
   argument