bitbake: create cooker object after cleaning the environment

bitbake: create cooker object after cleaning the environment

Previously, the cooker object was created before the environment was
cleaned, saving everything that was in the environment and dumping
into the run scripts.

The patch ensures that the cooker gets a cleaned environment when it's
created.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>