rust: add rust/.kunitconfig

Add a .kunitconfig file with the required configuration options to
allow to easily run the KUnit tests without adding them manually:

	$ ./tools/testing/kunit/kunit.py run --kunitconfig=rust \
	  --make_options LLVM=1 --arch=x86_64

Note that "CONFIG_UML" is set to "n" because UML is not working at the
moment [1].

[1] Rust-for-Linux#881

Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>

rust/.kunitconfig

@@ -0,0 +1,6 @@
+CONFIG_KUNIT=y
+CONFIG_RUST=y
+CONFIG_RUST_KERNEL_KUNIT_TEST=y
+
+# UML is not supported at the moment
+CONFIG_UML=n