Since bitbake now supports an official method to inject python modules,
switch to it.
Anyone using OE_EXTRA_IMPORTS will need to adjust their code accordingly,
probably switching to their own module namespace.
Also switch to using BB_GLOBAL_PYMODULES to list the global modules
to import.
(From OE-Core rev: 1f56155e91da2030ee0a5e93037c62e1349ba89f)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>