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
1 file changed