sca: eclair: Make ECLAIR project root configurable.

The project root was set unconditionally as the variable ZEPHYR_BASE,
but users developing a Zephyr application may wish to override the project
root used by ECLAIR as the base directory of their projects, so that
all paths processed by the tool are relative to that directory.

- A new cmake variable ECLAIR_PROJECT_ROOT is introduced;
- additional messages to output this information before the build starts.

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