Make bootclasspath srcjar available to make

Create a genrule, which creates an entry in Android.mk.

Bug: 151360309
Test: m platform-bootclasspath.srcjar
Change-Id: I23e62e72d369059a41ddd566c378b9a8bf0ad6ec

boot/Android.bp

@@ -192,6 +192,13 @@ custom_platform_bootclasspath {
     ],
 }
 
+genrule { // This module exists to make the srcjar output available to Make.
+    name: "platform-bootclasspath.srcjar",
+    srcs: [":platform-bootclasspath{.srcjar}"],
+    out: ["platform-bootclasspath.srcjar"],
+    cmd: "cp $(in) $(out)",
+}
+
 platform_systemserverclasspath {
     name: "platform-systemserverclasspath",
 }