targetcontrol.py: Add a classmethod to get extra files needed by the target controllers
YB: #6254
Add a new classmethod that can be used by outside scripts to get the extra files needed by the target controllers.
An outside script can predict rootfs, manifest and kernel files needed by a target controller, but sometimes there are other files needed.
Signed-off-by: Corneliu Stoicescu <corneliux.stoicescu@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
