mtd: add support for bad blocks in NAND flash

NAND flash is very likely to contain bad blocks.

Currently, mtd and therefore sysupgrade fails when it encounters a single bad block, potentially leaving an unbootable system.

This patch allows the mtd utility to skip bad blocks in NAND flash and complete sysupgrade successfully.

Patch by: Matthew Redfearn <matt.redfearn@nxp.com>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40021 3c298f89-4303-0410-b956-a3cf2f4a3e73