If this is running the first flash off of factory firmware, it is often useful to re-flash OpenWrt again from OpenWrt (using sysupgrade -n) to be sure of a good clean install.
"jffs not ready yet" suggests there is a problem formatting the rootfs_data flash. Check dmesg for jffs related errors. I would suggest a re-install then wait at least 5 minutes after it boots for the jffs to be built.
Every time when I run mount_root I get the following (in dmesg):
[ 49.738285] jffs2_scan_eraseblock(): End of filesystem marker found at 0x10000
[ 49.752786] jffs2_build_filesystem(): unlocking the mtd device...
[ 49.752792] done.
[ 49.768939] jffs2_build_filesystem(): erasing all blocks after the end marker...
[ 121.382895] done.
[ 121.401722] jffs2: notice: (473) jffs2_build_xattr_subsystem: complete building xattr subsystem, 0 of xdatum (0 unchecked, 0 orphan) and 0 of xref (0 dead, 0 orphan) found.
[ 121.474906] overlayfs: upper fs does not support tmpfile.
Please, help me. I don't understand what I am doing wring!