Due to the recursive ubi_thread implementation in the barebox, a large
amount of wear-leveling can lead to a stack overflow.

This was observed during extensive ubi stress tests with the linux
kernel and periodic power cycles. We found that if the wear-leveling
threshold is exceeded and a large amount of erase blocks need
wear-leveling the stack can overflow.

The hardware used to observe this was a phyCORE-i.MX 6 with 1GB NAND flash.

As the kernel is perfectly capable of handling wear-leveling we can
disable wear-leveling in the barebox by maxing out the threshold and
removing its Kconfig option.

Signed-off-by: Stefan Riedmueller <s.riedmueller@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>