fstools: fix multiple extroot problems

- 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>

SVN-Revision: 44535