mpc85xx: generic: Check kernel size for the TL-WDR4900

TP-Link TL-WDR 4900 have u-boot with read-only env.
Boot command read only 0x29F000 data from flash.
Bigger images causes crc error. It can't be changed.

This patch add kernel size checking.

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com> [utilize KERNEL_SIZE]

target/linux/mpc85xx/image/Makefile

@@ -51,6 +51,7 @@ define Device/tl-wdr4900-v1
   TPLINK_HWID := 0x49000001
   TPLINK_HWREV := 1
   TPLINK_FLASHLAYOUT := 16Mppc
+  KERNEL_SIZE := 2684k
   KERNEL_NAME := cuImage.tl-wdr4900-v1
   KERNEL_INITRAMFS :=
   SUPPORTED_DEVICES:=tl-wdr4900-v1 tplink,tl-wdr4900-v1