tools: kwbimage: Fix dumping NAND_BLKSZ

kwbimage nandblocksize field is in 64 kB unit, but NAND_BLKSZ command
expects it in bytes. So do required unit conversion.

Also zero value in nandblocksize field has special meaning. When this field
is set to zero, the default block size is used. This default size is
defined by the NAND flash page size (16 KB for a 512B page or small page
NAND and 64 KB for a large page NAND flash).

Fixes: 1a8e6b63 ("tools: kwbimage: Dump kwbimage config file on '-p -1' option")
Signed-off-by: Pali Rohár <pali@kernel.org>