Support wifi as default network interface

[ccli8]

Summary of changes

This supports configuring WiFi as default network interface. This support requires overriding weak symbol WiFiInterface::get_default_instance when e.g. "esp8266.provide-default: true" is specified in mbed_app.json. The override will fail because WiFi objects are archived in static library libmbed-wifi.a and don't participate in linking as expected, dependent on linker and library link order. To fix this, the override objects are extracted and passed to linker command line so that they always participate in the linking.

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Pull request type

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

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[multiplemonomials]

Hmm, I'm confused why this is needed... I have successfully used Arduino boards with Mbed CE that use Cypress WHD wifi modules, and I do not believe those needed anything special to get the default network interface to work

[multiplemonomials]

Actually no, you're right, this is an issue as the weak symbol override is defined in a .a file. How the heck did this work before?

Anyway, I guess we either need to do this, or we need to do something like move the definitions of WiFiInterface::get_target_default_instance() from the wifi drivers into NetworkInterfaceDefaults.cpp.

[ccli8]

Actually no, you're right, this is an issue as the weak symbol override is defined in a .a file. How the heck did this work before?

In Mbed CLI 2, mbed-wifi is interface library, not static library. It is e.g. ESP8266Interface.cpp.obj for ESP8266 WiFi not libmbed-wifi.a which appears in the linker command line. ESP8266Interface.cpp.obj can override weak symbol anyway.

or we need to do something like move the definitions of WiFiInterface::get_target_default_instance() from the wifi drivers into NetworkInterfaceDefaults.cpp.

This would be more concise. But if the override comes from another wifi library out of mbed-os tree, then it also cannot override unless it can apply either of above approaches.

[ccli8]

Fixed via #377