Avoid inheriting PRIVATE_BOOTCLASSPATH from moduels that depend on api-stubs-timestamp

The inherited value has caused build breakage on Mac build.

Change-Id: I8a1b1d6c6d411f6aaaee2d8dbd0297e3d60ab5c2
diff --git a/core/droiddoc.mk b/core/droiddoc.mk
index 0b8cd24..ba4857b 100644
--- a/core/droiddoc.mk
+++ b/core/droiddoc.mk
@@ -57,6 +57,7 @@
 $(full_target): PRIVATE_CLASSPATH:=$(LOCAL_CLASSPATH)
 full_java_lib_deps :=
 
+$(full_target): PRIVATE_BOOTCLASSPATH :=
 ifeq ($(BUILD_OS),linux)
 # You have to set bootclasspath for javadoc manually on linux since Java 6.
 host_jdk_rt_jar := $(dir $(HOST_JDK_TOOLS_JAR))../jre/lib/rt.jar