raviole: clean up kernel variables

1) make TARGET_KERNEL_DIR assignment conditional
2) add TARGET_BOARD_KERNEL_HEADERS

Bug: 198682676
Change-Id: Ica7d816bb421341f3620e156796ef611f437d688
diff --git a/device-slider.mk b/device-slider.mk
index f4e6b57..8acb9ff 100644
--- a/device-slider.mk
+++ b/device-slider.mk
@@ -14,7 +14,8 @@
 # limitations under the License.
 #
 
-TARGET_KERNEL_DIR := device/google/raviole-kernel
+TARGET_KERNEL_DIR ?= device/google/raviole-kernel
+TARGET_BOARD_KERNEL_HEADERS := device/google/raviole-kernel/kernel-headers
 
 $(call inherit-product-if-exists, vendor/google_devices/raviole/prebuilts/device-vendor-slider.mk)
 $(call inherit-product-if-exists, vendor/google_devices/gs101/prebuilts/device-vendor.mk)