bootstd: Add a test for bootmeth_cros

The ChromiumOS bootmeth has no tests at present. Before adding more
features. add a basic test.

This creates a disk which can be scanned by the bootmeth, so make sure
things work. It is quite rudimentary, since the kernel is faked, the root
disk is missing and there is no cmdline stored.

Enable the bootmeth for snow so it can build the unit test.

Signed-off-by: Simon Glass <sjg@chromium.org>