test/py: Allow tests to control the sandbox device-tree file

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>

test/py/conftest.py

@@ -179,6 +179,7 @@ def pytest_configure(config):
     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',