BBHandler: Make inherit calls more directly
    
    Rather than recursing into the conf handler code, simply call into
    the parse code directly when inheriting files as we've already resolved
    the paths and don't need anything the other codepath brings. This
    makes the codepath clearer at the expense of some slight duplication.
    
    Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>