sca: eclair: improve ECLAIR integration

Various improvements to the integration files in order to obtain correct
static analysis results:
- variable `ECLAIR_WORKSPACE' is set to the build output directory, to
  allow multiple ECLAIR analyses running in parallel on the same machine
  without conflicts;
- fix the evaluation order of rulesets to allow a user-provided ruleset to
  be selected without a cmake options file that undefines other
  ECLAIR_RULESET_* variables;
- add a clean-only frame to initiate the analysis and a project
  frame to end it.

Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>