[API] First attempt at describing a low level messaging API
[API] Remove unneeded Return-code in messaging-api ASN.1
[API] In Makefile, use a more generic rule to validate ASN.1 files
[API] Add Eurobalise dataflow extracted from Alstom's dataflow description
[API] Add ASN.1 description for BTM interface
[API] Add in description document Balise interface
[API] Update API description PDF