overview-manual, mega-manual: Updated image generation image

The figure for image generation of the BitBake process
needed some enhancing.

(From yocto-docs rev: fc978663a7962d1515d4f47abe8ae522b9e24ce1)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>