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>