nandwrite: add --input-{skip,size} options

nandwrite: add --input-{skip,size} options

If you have a file image and want to copy sub-portions out and into
NAND, there's no easy way to do that.  You can use dd to extract it
to a temp file, or pipe it to nandwrite 1 page at a time.  Both suck.

Add two new flags to explicitly set the size and offset of the input
file.  Seeking stdin isn't currently supported as I'm not sure it's
necessary.  It wouldn't be hard to add though...

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>