fstools: fix 'firstboot' on unmounted UBIFS overlay

fstools: fix 'firstboot' on unmounted UBIFS overlay

The usual OpenWrt-way of writing the JFFS2-marker in order to have
a filesystem erased at the next boot fails on UBIFS volumes due to
UBI being a different beast when it comes to writing.
As truncating a UBIFS volume only takes a few milliseconds and has the
desired effect of wiping-out all content of that volume, just do that
instead.

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