[Tizen] Split the package into crosswalk-bin and crosswalk-libs.

crosswalk.spec has been split into crosswalk-bin.spec and
crosswalk-libs.spec, each of which generates a separate RPM package:

* crosswalk-bin: The actual Crosswalk web runtime with all the xwalk*
  binaries and supporting files (systemd and D-Bus service files, for
  example).

* crosswalk-libs: The Chromium shared libraries and supporting
  files (such as icudtl.dat) required by the crosswalk-bin RPM. It has
  no dependencies on any Tizen-specific packages/libraries.

The main goal of the split is to be able to build them
independently (and have them as separate projects in build.tizen.org).
This way, changes to Tizen libraries (or to src/xwalk itself) only cause
crosswalk-bin to be rebuilt. Building crosswalk-bin should take only a
few minutes (instead of the hours it took to build the whole Crosswalk
RPM before).

The README in packaging/ has been updated, and the TL;DR; version is
that one must now specify which package needs to be built when calling
gbs build:

  $ gbs build --spec crosswalk-bin.spec # or crosswalk-libs.spec

Building crosswalk-libs requires some trickery:

* First of all, we do not want to depend on any Tizen package, otherwise
  changes to them in tizen.org would automatically trigger a
  crosswalk-libs rebuild that can potentially take hours as described
  above.
  We solve this by introducing a "building_crosswalk_bin" variable (its
  default value is 1) to build/system.gyp, which already centralises all
  the detection of additional package dependencies and pkg-config calls.
  If it is 0, pkg-config will not be called at all in targets such as
  "tizen", which is what we want and do in crosswalk-libs.spec.

* Secondly, we need to build all the shared libraries that the Crosswalk
  binaries need. We cannot hardcode this list, so the only reliable and
  future-proof way of doing it is to use gyp's dump_dependency_json
  generator and parse the dependency graph it generates to determine
  which targets in src/ (but not src/xwalk) need to be built. This is
  what the newly-added print-chromium-deps.py script does.

As for crosswalk-bin, it requires some invasive changes to Chromium's
build system to avoid building all the heavy Chromium targets such as
"contents" or "blink_web". The bulk of the change is in
crosswalk-do-not-build-several-chromium-dependencies.patch. This patch
changes several targets in the build system so that they do not build or
generate any files and just pass "-l<target>" to the linker. In other
words, we trick the build system into assuming nothing needs to be built
and using the shared libraries crosswalk-libs has installed into
%{_libdir}/xwalk/lib instead. As the big description in the patch
mentions, maintaining this patch up-to-date across Chromium releases
requires manual intervention since targets and dependencies are very
likely to change every time. The amount of pain in the arse this is
going to represent over time is still to be determined.

Another noteworthy change is that libxwalk_backend_lib.so's installed
location has changed from %{_libdir}/xwalk to %{_libdir}/lib since we
need the latter anyway for all the other shared libraries. We then
employ a small amount of shell scripting to make sure it is not part of
the list of files we want in the crosswalk-libs RPM.

BUG=XWALK-2571