net: make dhcp_run() common for NET and NET_LWIP

There are currently two implementations of dhcp_run(): one in cmd/net.c
for NET and one in net/lwip/dhcp.c for NET_LWIP. There is no
justification for that. Therefore, move the NET version into
net/net-common.c to be used by both stacks, and drop the NET_LWIP
version which by the way does not look totally correct.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Suggested-by: Tom Rini <trini@konsulko.com>
Acked-by: Benjamin Hahn <B.Hahn@phytec.de>

include/net-common.h

@@ -479,6 +479,16 @@ int net_loop(enum proto_t protocol);
  */
 int dhcp_run(ulong addr, const char *fname, bool autoload);
 
+/**
+ * do_dhcp - Run the dhcp command
+ *
+ * @cmdtp: Unused
+ * @flag: Command flags (CMD_FLAG_...)
+ * @argc: Number of arguments
+ * @argv: List of arguments
+ * Return: result (see enum command_ret_t)
+ */
+int do_dhcp(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[]);
 
 /**
  * do_ping - Run the ping command