[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.