Bug 1241022 - Move JAR_MANIFESTS. r=gps

JAR_MANIFESTS affects the libs tiers through config/rules.mk rules.
While we could move the rules in the backend, they are too complex to
just do that now.

python/mozbuild/mozbuild/backend/recursivemake.py

@@ -537,6 +537,7 @@ class RecursiveMakeBackend(CommonBackend):
             self._process_test_harness_files(obj, backend_file)
 
         elif isinstance(obj, JARManifest):
+            self._no_skip['libs'].add(backend_file.relobjdir)
             backend_file.write('JAR_MANIFEST := %s\n' % obj.path.full_path)
 
         elif isinstance(obj, Program):

python/mozbuild/mozbuild/frontend/context.py

@@ -1354,7 +1354,7 @@ VARIABLES = {
         JAR manifests are files in the tree that define how to package files
         into JARs and how chrome registration is performed. For more info,
         see :ref:`jar_manifests`.
-        """, 'libs'),
+        """, None),
 
     # IDL Generation.
     'XPIDL_SOURCES': (StrictOrderingOnAppendList, list,