overview-manual, mega-manual: Updated the Images figure

I needed to add some italic formatting for variable type strings.

(From yocto-docs rev: afc6d2d10081f9e9d940075c03b3e704b37cf13b)

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