kernel: rootfs auto-mount on ubi

kernel: rootfs auto-mount on ubi

Similar to the rootfs hacks on NOR flash devices, this series
introduces support for auto-attaching (ubi device), auto-creating
(ubiblock device) and mounting the "rootfs" (ubifs or squashfs)
volume.
This is needed so OpenWrt can start without relying on the bootloader
to pass the ubi.mtd, ubi.block, rootfs and rootfstype parameters, but
instead auto-detect the root filesystem according to a simple convention.

OpenWrt-specific:
490-ubi-auto-attach-mtd-device-named-ubi-or-data-on-boot.patch
491-ubi-auto-create-ubiblock-device-for-rootfs.patch
492-try-auto-mounting-ubi0-rootfs-in-init-do_mounts.c.patch
493-ubi-set-ROOT_DEV-to-ubiblock-rootfs-if-unset.patch

sent upstream:
552-ubifs-respect-silent-mount-flag.patch
http://lists.infradead.org/pipermail/linux-mtd/2014-May/053893.html

v2: actually retry with MS_RDONLY when mounting read-only ubifs root

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41119 3c298f89-4303-0410-b956-a3cf2f4a3e73