IMAGE_SIZE was previously set to kernel1 + ubi size = 256768k, now
kernel1 is 6MB adjust this value to add 3072k to total image size.
Signed-off-by: Tim Lunn <tim@feathertop.org>
Link: https://github.com/openwrt/openwrt/pull/15194
Signed-off-by: Petr Štetiar <ynezz@true.cz>