documentation/dev-manual/figures: Three figures for kernel added.

these are new figures to support the "Modifying the Kernel" conceptual
section.

(From yocto-docs rev: 1a0ecc104479b54a8122e3de2b30694fac8d7e73)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>