Re-apply ndk-docs rule.

This time wrapped with a check for the docs directory for unbundled
branches.

Test: make checkbuild
Bug: None
Change-Id: If80f0a03850d6cad3eab6d759af02ff2a7870974
diff --git a/core/envsetup.mk b/core/envsetup.mk
index c99c70f..4e3db0c 100644
--- a/core/envsetup.mk
+++ b/core/envsetup.mk
@@ -343,6 +343,7 @@
 PRODUCT_OUT := $(TARGET_PRODUCT_OUT_ROOT)/$(TARGET_DEVICE)
 
 OUT_DOCS := $(TARGET_COMMON_OUT_ROOT)/docs
+OUT_NDK_DOCS := $(TARGET_COMMON_OUT_ROOT)/ndk-docs
 
 BUILD_OUT_EXECUTABLES := $(BUILD_OUT)/bin
 SOONG_HOST_OUT_EXECUTABLES := $(SOONG_HOST_OUT)/bin