layerscape: make uImage with zImage for 32-bit kernel

uImage should be made with zImage. This patch is to support
making uImage with zImage.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>

target/linux/layerscape/Makefile

@@ -10,7 +10,6 @@ BOARD:=layerscape
 BOARDNAME:=NXP Layerscape
 DEVICE_TYPE:=developerboard
 KERNEL_PATCHVER:=4.4
-KERNELNAME:=Image dtbs
 FEATURES:=squashfs nand usb pcie gpio
 SUBTARGETS:=armv8_64b armv8_32b
 MAINTAINER:=Yangbo Lu <yangbo.lu@nxp.com>

target/linux/layerscape/armv8_32b/target.mk

@@ -8,6 +8,7 @@
 ARCH:=arm
 BOARDNAME:=ARMv8 32-bit based boards
 CPU_TYPE:=cortex-a9
+KERNELNAME:=zImage dtbs
 
 define Target/Description
 	Build firmware images for NXP Layerscape ARMv8 32-bit based boards.

target/linux/layerscape/armv8_64b/target.mk

@@ -7,6 +7,7 @@
 
 ARCH:=aarch64
 BOARDNAME:=ARMv8 64-bit based boards
+KERNELNAME:=Image dtbs
 
 define Target/Description
 	Build firmware images for NXP Layerscape ARMv8 64-bit based boards.

target/linux/layerscape/image/Makefile

@@ -33,15 +33,17 @@ endef
 define Device/Default
   PROFILES = Default
   FILESYSTEMS := squashfs
-  KERNEL := kernel-bin | gzip | uImage gzip
   DEVICE_DTS :=
   IMAGES = firmware.bin
 
 ifeq ($(SUBTARGET),armv8_64b)
+  KERNEL := kernel-bin | gzip | uImage gzip
   KERNEL_LOADADDR = 0x80080000
   KERNEL_ENTRY_POINT = 0x80080000
 endif
 ifeq ($(SUBTARGET),armv8_32b)
+  KERNEL := kernel-bin | uImage none
+  KERNEL_NAME := zImage
   KERNEL_LOADADDR = 0x80008000
   KERNEL_ENTRY_POINT = 0x80008000
 endif