Use statically defined sizes for kernel and rootfs mtd partitions.
Vendor upgrade script writes both firmware parts independently which
ends up in a gap between kernel and rootfs images. This results in
incorrectly calculated rootfs_data start offset.
Also, fix IMAGE_SIZE, DEVICE_PACKAGES and drop redundant KERNEL
definition.
Fixes FS#835
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>
656ed7544f