The ConfigParsed event is raised multiple times during a build which resulted
in the sanity warning appearing up to 4 times per build. Instead we should be
watching for the SanityCheck event (this is what the sanity checks in oe-core
watch for).

Signed-off-by: Paul Barker <pbarker@toganlabs.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>