This is described on this mailing list, but here’s the short of it in case that page ever dies:
If /dev/mtdblock0 doesn’t exist, do the following
mknod /dev/mtdblock0 b 31 0
Then do the following. The kernel modules can be enabled under Device Drivers->MTD. Make sure to get them all.
modprobe mtd # get this when enabling MTD modprobe jffs2 # under Filesystems, Misc modprobe mtdram # under MTD section modprobe mtdchar # under MTD section modprobe mtdblock # under MTD section dd if=your.jffs2 of=/dev/mtd0 mount -t jffs2 /dev/mtdblock0 /your-mount-point