Currently, anything whitelisted in the environment makes it into the worker
processes. This is undesireable and the worker environment should be as
clean as possible. This patch adapts bitbake sosme variables are loaded into
bitbake's datastore but not exported by default. Any variable can be exported
by setting its export flag.

Currently, this code only finalises the environment in he worker as doing so
in the server means variables are unavailable in the worker. If we switch
back to fork() calls instead of exec() this code will need revisting.

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