image.bbclass: Add support to build the SDK in parallel with the image

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.)

(From OE-Core rev: a0de2a56f19ae4d8cd88e46e96917a7a019fe1ab)

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>