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>

target/linux/ath79/image/nand.mk

@@ -88,12 +88,15 @@ define Device/glinet_gl-ar750s-common
   VID_HDR_OFFSET := 2048
 endef
 
+# NB: The kernel size is intentionally restricted at this time; see commit message
 define Device/glinet_gl-ar750s-nor-nand
   $(Device/glinet_gl-ar750s-common)
   DEVICE_VARIANT := NOR/NAND
   BLOCKSIZE := 128k
+  GL_UBOOT_UBI_OFFSET := 2048k
   IMAGES += factory.img
-  IMAGE/factory.img := append-kernel | pad-to $$$$(KERNEL_SIZE) | append-ubi
+  IMAGE/factory.img := append-kernel | pad-to $$$$(GL_UBOOT_UBI_OFFSET) | \
+			append-ubi | check-kernel-size $$$$(GL_UBOOT_UBI_OFFSET)
   IMAGE/sysupgrade.bin := sysupgrade-tar | append-metadata
   SUPPORTED_DEVICES += glinet,gl-ar750s-nor
 endef