net: make 'dhclient' behave like old 'dhcp' command

Picotcp tends to work in an asynchronious way.

E.g. for dhclient we have to use "pico_dhcp_initiate_negotiation()"
(to start dhcp client background task) and the special handler
(callback function) "callback_dhcpclient()".

But the 'dhcp' command in barebox works in very simple way:

  * user types 'dhcp' in the barebox shell command line;
  * the 'dhcp' command takes all control until it gets the positive
    or a negative result;
  * the control is returned to the barebox shell.

To use asynchronious-oriented picotcp functions in syncronious-oriented
barebox workflow we have to work out necessary picotcp usage template.

We can organise eternal cycle which ends on picotcp dhcp client fail
or on user request.

Latest picotcp introduces pico_dhcp_client_abort() function
so use it for interrupting picotcp dhcp client background work
on user request (ctrl-c).

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
net: make 'picoping' behave like traditional 'ping' command

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>