nanddump: add options --bb=skipbad and padbad

nanddump: add options --bb=skipbad and padbad

In mtd-utils, the bad block options changed in favor of
--bb=[skipbad|padbad|dumpbad] and omitbad has been removed.

This patch add the --bb=skipbad and padbad methods to busybox' nanddump.
padbad is the current default behaviour.
The difference between skipbad and omitbad is this one:
On a 16K block NAND, if the 1st block of mtd0 is bad, we'll have:
nanddump  -b -l 16384 /dev/mtd0 | wc -c
0
nanddump  --bb=skipbad -l 16384 /dev/mtd0 | wc -c
16384 <- data from 1st good block

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>