build: define check-kernel-size to remove unflashable images

build: define check-kernel-size to remove unflashable images

Certain boards have limitations on U-Boot that prevent flashing
of images where the kernel size exceeds a threshold, yet
sysupgrade can sucessfully manage larger kernels. The current
check-size will remove the target artifact if its total size
exceeds the threshold. If applied after append-kernel,
it will remove the kernel, but the remaining image-assembly
steps will continue, resulting in an image without a kernel
that is likely unbootable.

By defining check-kernel-size, it is now possible to prevent release
of such unbootable images through a construct similar to:

  IMAGE/factory.img := append-kernel | pad-to $$$$(GL_UBOOT_UBI_OFFSET) | \
    append-ubi | check-kernel-size $$$$(GL_UBOOT_UBI_OFFSET)

Cc: Chuanhong Guo <gch981213@gmail.com>
Signed-off-by: Jeff Kletsky <git-commits@allycomm.com>