Add an emulation of an SD card to sandbox, allowing MMC to be used in tests.
The emulation is very simple, supporting only card detection and reading
test data.
Signed-off-by: Simon Glass <sjg@chromium.org>
Add a test to confirm that we can probe this device. Since there is no
MMC stack support in sandbox at present, this is as far as the test goes.
Signed-off-by: Simon Glass <sjg@chromium.org>