kernel.mk: fix kernel targets for arm64

Use the Image.gz kernel target for arm64 to be able append DTB to kernel
bin at the end.

BUG: 27674989

Change-Id: I795db9dd235d14d679904e9e3d8074f19ee757f1
Signen-nff-by: Dzmitry Yatsushkevich <dmitryya@nestlabs.com>
diff --git a/kernel.mk b/kernel.mk
index 73afb39..609d410 100644
--- a/kernel.mk
+++ b/kernel.mk
@@ -65,7 +65,7 @@
 KERNEL_CROSS_COMPILE := $(KERNEL_TOOLCHAIN_ABS)/aarch64-linux-androidkernel-
 KERNEL_SRC_ARCH := arm64
 KERNEL_CFLAGS :=
-KERNEL_NAME := Image
+KERNEL_NAME := Image.gz
 else ifeq ($(TARGET_KERNEL_ARCH), i386)
 KERNEL_CROSS_COMPILE := $(KERNEL_TOOLCHAIN_ABS)/x86_64-linux-androidkernel-
 KERNEL_SRC_ARCH := x86