Fix TARGET_PREBUILT_KERNEL

Change-Id: I26a54d9b92362f9c21198f1c8823fc36dbb6d98f
Signed-off-by: Michael Wright <michaelwr@google.com>
diff --git a/device.mk b/device.mk
index 365d502..ba7d63e 100644
--- a/device.mk
+++ b/device.mk
@@ -21,7 +21,11 @@
 
 DEVICE_PACKAGE_OVERLAYS := device/asus/flo/overlay
 
+ifeq ($(TARGET_PREBUILT_KERNEL),)
 LOCAL_KERNEL := device/asus/flo-kernel/kernel
+else
+LOCAL_KERNEL := $(TARGET_PREBUILT_KERNEL)
+endif
 
 PRODUCT_COPY_FILES := \
 	$(LOCAL_KERNEL):kernel