cmd_nand: some infrastructure fixes and refactoring

- If the current device is overridden by a named partition,
  - update the caller's pointer/index, rather than copy over the
    nand_info struct, and
  - be sure to call board_nand_select_device even when the device
    is overridden by a named partition.
- Support 64-bit offsets/sizes in a few more places.
- Refactor arg_off_size for added readability and flexibility,
  and some added checks such as partition size.
- Remove redundant check for bad subcommands -- if there's no match
  it'll print usage when it gets to the end anyway.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Tested-by: Ben Gardiner <bengardiner@nanometrics.ca>