kernel: use dtc from prebuilt

dtc from OUT_DIR might not be available, so use the prebuilt one instead.

Also, clean-up a little bit and follow the same logic as prebuilt make binary.

Change-Id: I4dbf6233db98d5528f0d3890141e46d7a6d8e1c3
diff --git a/config/BoardConfigSoong.mk b/config/BoardConfigSoong.mk
index 8f3b724..7eba1b6 100644
--- a/config/BoardConfigSoong.mk
+++ b/config/BoardConfigSoong.mk
@@ -5,6 +5,7 @@
     KERNEL_ARCH \
     KERNEL_BUILD_OUT_PREFIX \
     KERNEL_CROSS_COMPILE \
+    KERNEL_DTC_CMD \
     KERNEL_MAKE_CMD \
     KERNEL_MAKE_FLAGS \
     PATH_OVERRIDE_SOONG \