test/py: test_efi_grub_net() requires OF_CONTROL

With CONFIG_OF_CONTROL environment variable $fdtcontroladdr is not defined
and test_efi_grub_net() fails.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/test/py/tests/test_efi_loader.py b/test/py/tests/test_efi_loader.py
index e105645..9465c28 100644
--- a/test/py/tests/test_efi_loader.py
+++ b/test/py/tests/test_efi_loader.py
@@ -170,6 +170,7 @@
     expected_text = 'Hello, world'
     assert expected_text in output
 
+@pytest.mark.buildconfigspec('of_control')
 @pytest.mark.buildconfigspec('cmd_bootefi')
 def test_efi_grub_net(u_boot_console):
     """Run the grub.efi binary via TFTP.