Add a phony target for the NDK sysroots.

`make ndk` will build all the NDK sysroots.

Test: make ndk
Bug: http://b/27533932
Change-Id: Ic2f5bf61f8911f7b8d08a473f2fe0576f3e06224
diff --git a/core/main.mk b/core/main.mk
index b99de49..59e9524 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -1125,4 +1125,7 @@
 tidy_only:
 	@echo Successfully make tidy_only.
 
+ndk: $(SOONG_OUT_DIR)/ndk.timestamp
+.PHONY: ndk
+
 endif # KATI