We can't access the emit() function of varhistory currently as the datastore parameter
isn't handled correctly, nor is the output stream. Add a custom wrapper for this
function which handles the two details correctly.
(Bitbake rev: ba0fa084ccd2b1ade96425d158fd31e49e42f286)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>