ifplugd: misc fixes

* Busybox provides ifplugd, so build after it and make the menu option
  conditional on BR2_PACKAGE_BUSYBOX_SHOW_OTHERS
* Style cleanup

[Peter: really make it depend on BR2_PACKAGE_BUSYBOX_SHOW_OTHERS]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>