VERSION file: master VERSION file is in base folder, symlinked from nx*/VERSION and hw/nxagent/VERSION.

  This commit removes the debian/VERSION file at makes it now unnecessary to
  copy/symlink the VERSION file at build time. These build scripts got adapted:

    debian/roll-tarballs.sh
    debian/rules
    nx-libs.spec

  Furthermore, all NX component now use the main VERSION file as reference.

    typechange: nxcomp/VERSION
    typechange: nxcompext/VERSION
    typechange: nxcompshad/VERSION
    typechange: nxproxy/VERSION