Improve AVH Board Layers for Corstone-300

[ReinhardKeil]

I believe we should only register header files that are user relevant (configure some options). The result is this file

Board.clayer.yml.txt

I suggest to make the vsi directory structure simpler with just:

vsi/include
vsi/source
vsi/python
vsi/README.md   # briefly explain the content

[MiloradCvjetkovic]

Addressed in 95d5cb7 .

[MiloradCvjetkovic]

This still needs support for SDSIO over VSI.

[furbanc]

VSI support added in 8c7e329b