mtdparts: new add.spread: add part skipping bad blocks

This patch adds a new 'mtdparts add' variant: add.spread. This command variant
adds a new partition to the mtdparts variable but also increases the partitions
size by skipping bad blocks and aggregating any additional bad blocks found at
the end of the partition.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
CC: Wolfgang Denk <wd@denx.de>
CC: Scott Wood <scottwood@freescale.com>