centos: clean up yum caches after installing packages

So that we can get smaller rootfs.

Signed-off-by: Peng Tao <bergwolf@gmail.com>

rootfs-builder/centos/rootfs_lib.sh

@@ -129,6 +129,7 @@ build_rootfs() {
 
 	DNF="${PKG_MANAGER} --config=$DNF_CONF -y --installroot=${ROOTFS_DIR} --noplugins"
 	$DNF install ${EXTRA_PKGS} ${PACKAGES}
+	$DNF clean all
 
 	[ -n "${ROOTFS_DIR}" ]  && rm -r "${ROOTFS_DIR}/var/cache/centos-osbuilder"
 }