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>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 211be39..7173f3d 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -2110,10 +2110,9 @@
 config CMD_TFTPBOOT
 	bool "tftp"
 	select PROT_UDP_LWIP if NET_LWIP
-	default n
+	default y
 	help
 	  tftpboot - load file via network using TFTP protocol
-	  Currently a placeholder (not implemented) when NET_LWIP=y.
 
 endif  # if CMD_NET