scripts: generate image info header file

This commit adds the `gen_image_info.py` script which supports creation
of a header file with image information from the EFL file.

This version populates the header file with:
- Number of segments in the image
- LMA address of each segment
- VMA address of each segment
- Size of each segment

The header file can be used by a secondary build system which needs this
information.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>