From: Tom Rini <trini@kernel.crashing.org>

The following patch fixes the 'znetboot' and 'znetbootrd' targets so that
they work again.

- Update the comments to reflect how things work with the correct
  usages now.

- Fix the znetboot / znetbootrd targets.  We now always set end-y,
  and use this to figure out what image will be tftpboot'ed.