cassini-[bsp,config]: Correct distro features for firmware builds

There is a mismatch between distro features assigned when building
platform firmware in Cassini and what is specified by the platform.

Rather than setting distro features in cassini.conf, removing them in
firmware.conf, and relying on the order in which bitbake hard assigns
values, use the `cassini` override to set distro features for the distro
only.

Signed-off-by: Adam Johnston <adam.johnston@linaro.org>