[Packaging] Introduce gbp-flat-tree.sh to simplify the Tizen build process.

TL;DR: one can simply do the following to build an RPM:
  cd /path/to/src/xwalk
  gbs build -A i586

We can get rid of the need for a separate git tree and the
generate-flat-tree.sh script by relying on the hooks provided by
git-buildpackage: the postexport hook that we use is called right after the
git repository's sources have been exported with `git archive`.

In this hook, we execute a new script, `gbp-flat-tree.sh`. It does pretty
much the same thing as generate-flat-script.sh, but automatically. That is,
it removes the tarball generated by `git archive` (since gbs build should be
called from xwalk/, it would only contain Crosswalk's contents anyway) and
replaces it with a new one with the contents of src/.

Since it is called at a stage in the whole build process that immediately
precedes the call to `/usr/bin/build`, we do not need to keep a separate
flat git tree with all our sources anymore; we are effectively bypassing
git-buildpackage's default export mechanism.

The downside of this approach (which is shared by the previous one with
`generate-flat-tree.sh`, anyway) is that all files in src/ are archived,
even if they are not part of any commit. In other words, we end up acting as
if "--include-all" was always passed to `gbs build`.

(Fun fact: the whole dance of calling `gclient recursive` twice with
different arguments and everything was unnecessary; one of the entries
reported by those calls is src/ itself, so we always ended up archiving
everything under it)
[Packaging] Accept other build directories to support incremental builds.

TL;DR: Specify a directory outside the RPM build area with the BUILDDIR_NAME
macro to have incremental builds:
  cd /path/to/src/xwalk
  gbs build -A i586 --define 'BUILDDIR_NAME /tmp/crosswalk-builddir'

A full rebuild was being performed in every call to `gbs build` due to two
issues:

1. rpmbuild erases the RPM build directory by default in the %setup stage of
   the spec file. This can be solved by passing -D to %setup, but...
2. osc-build (`/usr/bin/build`) removes the whole RPM build area (that is,
   `/home/abuild/rpmbuild`) by default unless we pass "--incremental" to
   `gbs build`. Unfortunately, suffice it to say that the implementation of
   that option in gbs leaves a lot to be desired and does not solve our
   problem.

This can be solved by exploiting the way gyp generates Makefiles: it is
possible to override the default value "out" of the `builddir_name` variable
and have out-of-source builds. We can thus point it to a location outside
`/home/abuild/rpmbuild` in the chroot and automatically preserve it across
builds, as the rest of the buildroot is kept by gbs by default.

We then allow the user to specify a different build directory with the
`BUILDDIR_NAME` RPM macro that can be passed to gbs as explained in the
beginning of the commit message. One of Chromium's tools, grit, also had to
be fixed to allow out-of-source builds to work correctly, which explains the
addition in DEPS.xwalk.

Another thing worth mentioning is that we now use the `-n` option of the
%setup macro in the spec file as the root directory of the source tarballs
do not have Crosswalk's version anymore: even if those two problems related
to the removal of the build directory above are worked around, a full
rebuild is triggered if the source directory gets changed (think of a
version number bump, for example). While at this, it is also worth bearing
in mind that a change in CFLAGS or CXXFLAGS also triggers a rebuild, so
calling make directly from inside the gbs chroot without using the same
compiler flags as the RPM build will also cause problems.

Last but not least, the generate-flat-tree.sh script and its instructions in
the README will be removed in a later commit, once we get more people to
test the new solution.
[Packaging] Update the README after the last two commits.