Use tftproot variable instead of hard-coding /tftpboot
Create #{tftproot}/windows-6.2/extra directory before using it
Fix typo in comment
Allow some supported OS to not have an /install dir

This is needed for Windows.
Put files in the right directory for Hyper-V

Do not use "#{tftproot}/windows-6.2" but ""#{os_dir}" which will work
both for Windows Server and Hyper-V Server.