[NAND] Bad block skipping for command nboot

The old implementation of command nboot does not support reading the image from
NAND flash with skipping of bad blocks. The patch implements a new version of
the nboot command: by calling nboot.jffs2 from the u-boot command line the
command will load the image from NAND flash with respect to bad blocks (by using
nand_read_opts()). This is similar to e.g. the NAND read command: "nand
read.jffs2 ...".

Signed-off-by: Thomas Knobloch <knobloch@siemens.com>
Signed-off-by: Stefan Roese <sr@denx.de>