ipq40xx: use zImage for EX6100v2 and EX6150v2

The NETGEAR EX61500v2 and EX6150v2 U-Boot does not support booting LZMA
compressed images. Currently, they are using GZIP compressed kernels,
which results in ledd flash being available to the root and overlay
filesystems.

Using a zImage results in a smaller kernel and therefore increases
available space for rootfs and overlayfs.

Size reduced: ~1.1 MiB

Signed-off-by: David Bauer <mail@david-bauer.net>

target/linux/ipq40xx/image/Makefile

@@ -43,9 +43,7 @@ define Device/UbiFit
 endef
 
 define Device/DniImage
-	KERNEL_SUFFIX := -fit-uImage.itb
-	KERNEL = kernel-bin | gzip | fit gzip $$(DTS_DIR)/$$(DEVICE_DTS).dtb
-	KERNEL_NAME := Image
+	$(call Device/FitzImage)
 	NETGEAR_BOARD_ID :=
 	NETGEAR_HW_ID :=
 	IMAGES := factory.img sysupgrade.bin