Add support for Windows Server 2012 R2
Share Windows files among Windows version

There's no need to copy the same files in many directories.
Make it possible to use several versions of Windows at the same time

We were limited by the fact that we hardcoded the version of Windows in
tftpd.conf. Instead of doing that, we use \x in tftpd.conf which matches
the IP address of the requesting host, and with this IP address, we
create a symlink to the correct windows directory.
Fix typos in comment
Do not require patch in adk-tools and create symlinks

To avoid requiring a change in adk-tools, let's just make the
version-specific extra directories symlinks to the windows-common extra
directory.
Create windows common directory/files before version-specific files
Merge pull request #326 from vuntz/win2012r2

Add support for Windows Server 2012 R2