Add register description generator in Tools/.
Use %lex-param instead of #define YYLEX_PARAM.

YYLEX_PARAM has been deprecated in version 1.875 and is not supported
anymore, we have to use %lex-param instead.
Mention the libjson license in README.md.
Integrate the build of regsgen2 in the main CmakeLists.txt.

We can now do `make regsgen2` and build this tool.