bitbake/bin/bitbake: Make sure we add all handlers from INHERIT

bitbake/bin/bitbake: Make sure we add all handlers from INHERIT

    The code had the assumption that at least one .inc
    file was included (always included). This is not true
    in the NSLu2 case. The code now makes sure we load each
    bbclass we have in the INHERIT line either directly or
    indirectly. And then we can safely add all event handlers.

    Todo: Add HANDLERS += to only load the HANDLERS and not
    all classes.