ensure_wear_leveling() is called at the end of ubi_wl_init() and may
come to the decision to scrub some blocks. In the Kernel this is done
in a separate thread, but in barebox we do this synchronously. The
problem is that during ubi_wl_init() the EBA system is not yet
initialized (ubi_eba_init() is not yet called), so the wear level worker
hits a NULL pointer deref when the fastmap needs to be updated and
ubi_write_fastmap() accesses vol->eba_tbl.

Solve this by honoring the ubi->thread_enabled flag which is only set
to true when UBI is sufficiently initialized. This means we now can
have multiple works queued, so we can no longer simply do one work when
queued, but instead have to continue to do work until all work is done.
The best place to do so is a ubi_thread() function which behaves similar
to the Kernel function with the same name, but is called synchronously
in barebox. To make sure that the initially queued works are done, the
call to (no-op) wake_up_process() at the end of ubi_attach_mtd_dev()
is replaced with a call to ubi_thread().

While at it also honor the ubi->ro_mode flag to make sure we do not do
any wear leveling work on readonly UBI devices.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>