Normally tests will run with the test.dtb file designed for this purpose.
However, the verified boot tests need to run with their own device-tree
file, containing a public key.
Make the device-tree file a config option so that it can be adjusted by
tests. The default is to keep the current behaviour.
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Teddy Reed <teddy.reed@gmail.com>
     ubconfig.board_type = board_type
     ubconfig.board_identity = board_identity
     ubconfig.gdbserver = gdbserver
+    ubconfig.dtb = build_dir + '/arch/sandbox/dts/test.dtb'
 
     env_vars = (
         'board_type',
 
             self.config.build_dir + '/u-boot',
             '-v',
             '-d',
-            self.config.build_dir + '/arch/sandbox/dts/test.dtb'
+            self.config.dtb
         ]
         return Spawn(cmd, cwd=self.config.source_dir)