uninative: Ensure uninative is enabled in all cases for BuildStarted event

Recent changes in bitbake mean the datastore is not always reset between
ConfigParsed and BuildStarted. This means in a fresh buiild, with memory
resident bitbake active, uninative may end up disabled.

Update the code so the enable code is always run at BuildStarted if
needed.

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