kernel: Move TARGET_KERNEL_ADDITIONAL_FLAGS to kernel task

 * Currently, because DTC_EXT make flag is in the
   kernel task the only way to override it is to
   have TARGET_KERNEL_ADDITIONAL_FLAGS come after.

Change-Id: If62ed979581dc07eafd4628ce2b2d799388d3704
diff --git a/build/tasks/kernel.mk b/build/tasks/kernel.mk
index 6d68cf1..104ffd9 100644
--- a/build/tasks/kernel.mk
+++ b/build/tasks/kernel.mk
@@ -21,6 +21,7 @@
 #
 # These config vars are usually set in BoardConfig.mk:
 #
+#   TARGET_KERNEL_ADDITIONAL_FLAGS     = Additional make flags, optional
 #   TARGET_KERNEL_CONFIG               = List of kernel defconfigs, first one being the base one,
 #                                          while all the others are fragments that will be merged.
 #                                          to main one in .config.
@@ -231,6 +232,10 @@
     KERNEL_MAKE_FLAGS += DTC_EXT=$(KERNEL_BUILD_OUT_PREFIX)$(DTC)
 endif
 
+ifneq ($(TARGET_KERNEL_ADDITIONAL_FLAGS),)
+    KERNEL_MAKE_FLAGS += $(TARGET_KERNEL_ADDITIONAL_FLAGS)
+endif
+
 # Internal implementation of make-kernel-target
 # $(1): output path (The value passed to O=)
 # $(2): target to build (eg. defconfig, modules, dtbo.img)