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.

--HG--
extra : rebase_source : 2521d49a1731b659dd720769e4685715925be590
Bug 774572 - Part 2: Define JAR_MANIFESTS in moz.build files; r=glandium

Every directory with a jar.mn now has JAR_MANIFESTS defined in its
moz.build file.

We also removed the may_skip special consideration of jar.mn files
because this information is now available during tier traversal by the
reader courtesy of the variables being present in moz.build files.

--HG--
extra : rebase_source : 21049b15e6bd9cf65b0805ccaccc4ba5aae93c98
extra : amend_source : 0b1ea866d725beef92d37c6f6d475369ac002e19