net: lwip: add TFTP support and tftpboot command

Implement do_tftpb(). This implementation of the tftp command
supports an optional port number. For example:

 tftp 192.168.0.30:9069:file.bin

It also supports taking the server IP from ${tftpserverip} if
defined, before falling back to ${serverip}.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

include/net-common.h

@@ -459,6 +459,10 @@ static inline struct in_addr env_get_ip(char *var)
 
 int net_init(void);
 
+/* NET compatibility */
+enum proto_t;
+int net_loop(enum proto_t protocol);
+
 /**
  * dhcp_run() - Run DHCP on the current ethernet device
  *