The Flash img API is used for writting image application image
to flash and is rather part of DFU than storage.
Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
Doxygen comments were existing, but they were not rendered anywhere in
the docs. The code is located under DFU, but I think it makes sense
to document it under storage, next to similar flash-related APIs.
Signed-off-by: Martin Jäger <martin@libre.solar>