Bug 774572 - Part 1: Support for defining JAR manifests in moz.build; r=glandium

JAR_MANIFESTS can now be defined in moz.build files. However, due to
limitations in rules.mk, only 1 file may be defined at a time. In the
future, this restriction will be lifted. But first, better support for
JAR manifests in the build config must be built.

rules.mk will be updated in the subsequent conversion patch so this
patch applied alone doesn't break the build.

build/docs/jar-manifests.rst

@@ -7,7 +7,8 @@ JAR Manifests
 JAR Manifests are plaintext files in the tree that are used to package chrome
 files into the correct JARs, and create
 `Chrome Registration <https://developer.mozilla.org/en-US/docs/Chrome_Registration>`_
-manifests. JAR Manifests are named ``jar.mn``.
+manifests. JAR Manifests are commonly named ``jar.mn``. They are
+declared in ``moz.build`` files using the ``JAR_MANIFESTS`` variable.
 
 ``jar.mn`` files are automatically processed by the build system when building a
 source directory that contains one. The ``jar``.mn is run through the