It depends what folder contains. if it only contains regular files (non-executables):
install -d ${pkgdir}/opt/
install -m 644 ${srcdir}/${pkgname}-${pkgver}/folder/* ${pkgdir}/opt/If it contains subdirectories:
install -d ${pkgdir}/opt/
cp -r ${srcdir}/${pkgname}-${pkgver}/folder/* ${pkgdir}/opt/