df.py: Add feature check for read-only-rootfs

If the target is using a read-only rootfs, the available space on '/'
will be zero. This will cause the test to incorrectly fail and skipping
seems appropriate in this case.

Signed-off-by: Wes Lindauer <wesley.lindauer@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>