image.bbclass: IMAGE_VARS->IMAGE_BOOT, make overridable

image.bbclass: IMAGE_VARS->IMAGE_BOOT, make overridable

IMAGE_VARS usage forces images to have init, dev management, login
management and other stuff. In cases where you don't need any of those
(like initramfs images or other special-purpose images) you need to
override 4 variables in order to get clean image that only installs
things from IMAGE_INSTALL.

So, this changes IMAGE_VARS to more meaningful IMAGE_BOOT and makes it
possible to build images without boot components with single
IMAGE_BOOT="".
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Koen Kooi <koen@openembedded.org>