build: image: move IMAGE_SIZE to image.mk

build: image: move IMAGE_SIZE to image.mk

IMAGE_SIZE is widely used in many targets. Declare it in the default template to
clean up redundant code. This also prevents deriving IMAGE_SIZE unintentionally
from the previously defined device.

While at it, remove duplicate KERNEL_SIZE declaration.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>