fstools: update to git HEAD

fstools: update to git HEAD

 f0fc66a libfstools: check for overlay mounting errors
 128ecaf Update / fix extroot comments
 8a0ba3b libfstools: get rid of "extroot_prefix" global variable
 649cd3f libfstools: use variable for overlay mount-point
 922f1b3 libfstools: avoid segfault in find_mount_point
 ce5eacb libfstools: mtd: improve error handling
 898b328 blockd: restore device_move semantics
 0917d22 block: don't probe mtdblock on NAND (with legacy exceptions)

Signed-off-by: Daniel Golle <daniel@makrotopia.org>