Currently implementations of the MTD api are selecting the module,
this makes it easy for default backends to be selected. But the
correct modeling is simply selecting the MTD api and then selecting
a backend. BOARDs providing one of the backends can expose this and
then that backend can be selected by default.
There is also currently nothing preventing from using the MTD api
with no backend since a mock backend can easily be provided as is
done in all mtd tests.
