BB: fstools: cumulative backport

BB: fstools: cumulative backport

- Adds support for /dev/vd* virtual io
- Adds support for overlayfs v23
- Fixes overlayfs mount on 3.18
- Make the block tool ignore case when comparing uuid strings
- Fixes sysupgrade on linux 3.18
- Support extroot/ubi
- Allows using UBIFS volume as overlay and adds support for Btrfs
- Fix exit code of uci-defaults script
- Support external overlays on non-MTD systems again
- Account for new directory structure of overlayfs partitions (additional upper/ component)
- Support executing block from either overlay or rom
- Support loading fstab from either overlay or rom
- Log extroot failures to dmesg

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>
Backport of r42812, r42890, r43090, r43368, r43480, r43485, r43717, r43868, r44180, r44535

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@44537 3c298f89-4303-0410-b956-a3cf2f4a3e73