package/ubox: fix jffs2 handling on MTD devices emulated by gluebi

package/ubox: fix jffs2 handling on MTD devices emulated by gluebi

The jffs2_ready() function in mount_root.c checks
the presence of various JFFS2 markers at the start
of a given MTD device. The function works on NOR
flashes because JFFS2 puts 'cleanmarker' nodes at
the start of freshly erased blocks.

However if jffs2 is used on a MTD device emulated
by the gluebi layer, the 'cleanmarker' nodes are
not present and the jffs2_ready() function fails.

Update the code to handle jffs2 correctly even on
MTD devices emulated by the gluebi layer.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38654 3c298f89-4303-0410-b956-a3cf2f4a3e73