image.bbclass: Add support to build the SDK in parallel with the image
When building an image recipe, you can now build a companion SDK by
calling the populate_sdk task:
bitbake -c populate_sdk core-image-minimal
Note: there are still issues w/ the SDK not working completely with
multilibs.
A lock is required between rootfs and populate_sdk activities to prevent
configuration file clashes and similar package management problems in ipk
and deb based systems. (RPM already had a lock for a different reason.)
Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
