tftp: don't reuse the buffer when the packet hasn't sent yet

Signed-off-by: Reto Achermann <reto.achermann@inf.ethz.ch>

tftpclient: converting to net sockets
net_sockets: adding local/remote addresses/ports to the net_socket structure
net,dhcp: adding a possibility of setting of static IP

Signed-off-by: Adam Turowski <adam.turowski@inf.ethz.ch>

libtftp: initial commit of a simple tftp client library

generic usage:
tftp_client_connect();
tftp_client_read_file();
tftp_client_disconnect();

Currently, timeouts are not handled.

Signed-off-by: Reto Achermann <reto.achermann@inf.ethz.ch>