bsp-guide: Updated BSP development flow figure.

This figure was out of date.

(From yocto-docs rev: 8b649ab3c4667a45f263818c2532b18f2dae9464)

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