build.py: Cleanup data store references

Clean up a number of old style accesses to the datastore.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
cooker: Ensure configuration matches variable tracking state

When we reset, the current tracking value may change unexpectedly. This
patch ensures that the default value is preserved over a datastore reset.

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