cmd_onenand.c: moved to standard subcommand handling

On the fly also fixed the following things:
- write help talked about a parameter oob, but that one was not used, so
  removed it from the help message.
- the test command also allowed a force subcommand but didn't use it.
  eliminated the code.
- do_onenand made static
- do_onenand contained
	int blocksize;
	...
	mtd = &onenand_mtd;
	this = mtd->priv;
	blocksize = (1 << this->erase_shift);
  As blocksize was not used the last two statements were unneeded so
  removed them.
  The first statement (mtd = ....) assigns to a global. Not sure if it
  is needed, and since I could not test this, left the line for now

Signed-off-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>