ac14xx: rephrase network boot config for development

- remove the builtin 'rootpath' spec (according to U-Boot project
  policy) and require user provided environments to contain these
- rephrase the evaluation of the 'muster_nr' approach which allows to
  quickly switch among several network boot setups (make the setting
  transparent when empty, resulting in default DULG behaviour)
- reduce the ARP timeout for faster network boot

Signed-off-by: Gerhard Sittig <gsi@denx.de>