This transformer reverts the effect of `split_sections`, which we intend
to use for platforms which don't support split sections.

In order to achieve this we have to modify the implemntation of the
split_sections transformer to store whether we are enabling
split_sections directly in the `Flavour` definition. This is because
otherwise there's no convenient way to turn off split_sections due to
having to pass additional linker scripts when merging objects.