ath79: GL-AR750S (NOR/NAND): limit factory.img kernel size to 2 MB

ath79: GL-AR750S (NOR/NAND): limit factory.img kernel size to 2 MB

The present U-Boot for GL-AR750S has a limit of 2 MB for kernel size.
While sysupgrade can manage kernels up to the present limit of 4 MB,
directly flashing a factory.img with a kernel size greater than 2 MB
through U-Boot will result in an unbootable device.

This commit uses the newly-introduced check-kernel-size build
operation to prevent the output of factory.img when the kernel
exceeds 2 MB in size, yet permits output of sysupgrade.img
as long as the kernel is within KERNEL_SIZE := 4096k

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