db845c.mk: set DB845C_KERNEL_DIR to use environment when specified

so that we could use the files in the dist directory
directly, without copying them into device/linaro/dragonboard-kernel

Test: export DB845C_KERNEL_DIR=<output/db845c/dist>
      source build/envsetup.sh
      lunch db845c-userdebug
      make -j$(nproc) droidcore

Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
Change-Id: Iacdfe3ca107bffc92679bdf9273b6c5e329b9557
diff --git a/db845c.mk b/db845c.mk
index e91d412..df377f7 100644
--- a/db845c.mk
+++ b/db845c.mk
@@ -1,7 +1,7 @@
 ifndef TARGET_KERNEL_USE
 TARGET_KERNEL_USE := 5.4
 endif
-DB845C_KERNEL_DIR := device/linaro/dragonboard-kernel/android-$(TARGET_KERNEL_USE)
+DB845C_KERNEL_DIR ?= device/linaro/dragonboard-kernel/android-$(TARGET_KERNEL_USE)
 
 # Inherit the full_base and device configurations
 $(call inherit-product, $(SRC_TARGET_DIR)/product/core_64_bit.mk)