cmsBuild: allow building subpackages

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
cmsBuild: fix %_builddir/files lines in SPEC

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
cmsBuild: create symlinks in $WORKDIR/RPMS/$SCRAM_ARCH for subpackages

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
cmsBuild: now install subpackages

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>