[FEATURE] Add support for more project types

[jgabaut]

The anvil-kern argument could be used to support more workflows and project types.

• At the moment, the strong link with autotools for the ambosoC kern could be relaxed to allow for a simple script entryname.
• A new "custom" kern could be added, intended to be a single process call expected to do the build, named explicitly by a new anvil key in the stego.lock.

Relevant amboso issue:

• issue

This is not to be considered for:

• [FEATURE] Feature freeze on amboso 2.1 #172

, since it will come as part of the 2.1 API