Use Image.gz for testing

The output of `make linux_image` is Image.gz, not Image.

Change-Id: I69fe3a56238968096229a27b36602282842ab2f6
diff --git a/Makefile b/Makefile
index 0f55b2a..c7a8db5 100644
--- a/Makefile
+++ b/Makefile
@@ -38,7 +38,7 @@
 ifdef KERNEL_IMAGE
 CUSTOM_KERNEL_IMAGE=1
 else
-KERNEL_IMAGE := $(LINUX_OUT)/arch/arm64/boot/Image
+KERNEL_IMAGE := $(LINUX_OUT)/arch/arm64/boot/Image.gz
 endif
 
 ##