ref-manual: Added KERNEL_IMAGE_BASE_NAME change to 2.2 migration

ref-manual: Added KERNEL_IMAGE_BASE_NAME change to 2.2 migration

The fact that the OpenEmbedded build system can now build
multiple image types caused the KERNEL_IMAGE_BASE_NAME variable
to have the KERNEL_IMAGETYPE portion removed.  This could cause
existing recipes that use the KERNEL_IMAGE_BASE_NAME variable
directly issues.  I added a section indicating that the user
should address those recipes to avoid problems.

(From yocto-docs rev: c433fca3840440c14bb364a51afc5c3c2bfc7c88)

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