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.

(From OE-Core rev: fea627022473cfb73299d0988628962ad8e80f89)

Signed-off-by: Corneliu Stoicescu <corneliux.stoicescu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/targetcontrol.py

@@ -72,6 +72,10 @@ class BaseTarget(object):
     def stop(self):
         pass
 
+    @classmethod
+    def get_extra_files(self):
+        return None
+
     @classmethod
     def get_image_fstype(self, d, image_fstypes=None):
         if not image_fstypes: