rpmbuild: remove unnecessary codes.
rpmbuild: remove unused "vmapp" targets.
rpmbuild: remove unnecessary "execscript" codes.
rpmbuild: remove unnecessary "setup-ci-env.sh" dependency codes.
rpmbuild: always set base_distro=centos.
rpmbuild: merge "yum_opts".
rpmbuild: remove unnecessary comments.
rpmbuild: update installation order in chroot.
rpmbuild: remove unnecessary "sync" commands.
rpmbuild: merge "yum_opts".
rpmbuild: remove unnecessary "cat" command.
rpmbuild: remove unnecessary "uname" command in chroot.
rpmbuild: remove unnecessary codes.
rpmbuild: always set base_distro_number=6.
rpmbuild: remove unnecessary codes.
rpmbuild: remove unnecessary codes.
rpmbuild: update code layout.
rpmbuild: dest_chroot_dir -> chroot_dir.
rpmbuild: remove unnecessary codes.
rpmbuild: base_chroot_dir -> distro_dir.