classes/conf: Add eventmasks for event handlers

classes/conf: Add eventmasks for event handlers

Now that bitbake supports masking events for event handlers, lets use
this so event handlers are only called for events they care about. This
lets us simplify the code indentation a bit at least as well as mildly
improving the event handling performance.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>