ath79: gl-ar750s: reduce kernel size to 2M in image Makefile

ath79: gl-ar750s: reduce kernel size to 2M in image Makefile

u-boot splits nand factory firmware at 2M offset, flash the first
part as kernel into spi nor and the other part as ubi into nand
flash. With previous commit increasing kernel size to 4M, generated
factory firmware is broken because ubi is at 4M offset.

This commit reduces kernel size definition to 2M in image Makefile,
producing proper factory image. Partition size in dts is kept
unchanged so that sysupgrade to a firmware with 2M+ kernel still
works.

Fixes: b496a2294c ("ath79: GL-AR750S: provide NAND support; increase kernel to 4 MB")
Reported-by: Jeff Kletsky <git-commits@allycomm.com>
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>