scripts: compliance: Resolve ZEPHYR_BASE in _main()

Instead of globally resolving ZEPHYR_BASE, move it to _main() so we can
catch potential exceptions when doing so and for consistency with the
rest of globals, like GIT_TOP. This also includes documenting a new
"<zephyr-base>" path hint for classes.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>