CMSIS-NN: Add dilation support for conv (#1377)

* Adds dilation support for convolution implementation.
* Adds corresponding unit tests for dilation.
* Fixes generate script bug that it was always regenerating
   data.
* DSP and scalar implementation are partly combined so scalar
   implementation now also uses im2col buffer.

Change-Id: If6c57594b5122827597529ab628586135ec975fe
Co-authored-by: Patrik Laurell