Add support for hostname request in lwIP interface

[guilhermerc]

Summary of changes

Add support for hostname request in lwIP interface.

There is a discussion about this functionality here: How to set device network hostname on MBED OS 6?

Impact of changes

Migration actions required

Documentation

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[X] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

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Reviewers

@pan-

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[guilhermerc]

Follows an usage example. It was tested on mbed LPC1768.

#include "EthernetInterface.h"
#include "mbed.h"

int main() {
    EthernetInterface net;
    char hostname[] = "example-hostname";
    nsapi_error_t nsapi_status;

    nsapi_status = net.set_hostname(hostname);
    assert(nsapi_status == NSAPI_ERROR_OK);
    debug("[main] net.get_hostname: %s\n", net.get_hostname());

    do {
        ThisThread::sleep_for(5s);
        nsapi_status = net.connect();
        debug("[main] net.connect rc: %d\n", nsapi_status);
    } while(nsapi_status != NSAPI_ERROR_OK &&
        nsapi_status != NSAPI_ERROR_IS_CONNECTED);

    while(true) {}
}

[multiplemonomials]

Does doing this in add_ethernet_interface() mean that you have to call set_hostname() before adding the Ethernet interface? What about if the user calls set_hostname() later? We should probably document this in the NetworkInterface comments.

[guilhermerc]

Yes, EthernetInterface::set_hostname should be called before EthernetInterface::connect (which calls LWIP::add_ethernet_interface). If it's called later, EthernetInterface::set_hostname will return NSAPI_ERROR_BUSY, as the interface is already initialized.

This behaves exact like EthernetInterface::set_mac_address.

[guilhermerc]

We can rewrite NetworkInterface::set_hostname comments to address this behavior:

     *  @retval         NSAPI_ERROR_BUSY if interface is already initialized

I just think we should change it in both NetworkInterface::set_hostname and NetworkInterface::set_mac_address comments for consistency. What do you think?