Provide default WiFi interfaces for Ublox and Realtek

[SeppoTakalo]

Description

To allow application to choose default WiFi interface by calling WiFiInterface::get_default_instance() provide its implementation on these two targets.

Partially fixes:
#7257
#7246

Pull request type

[X] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[SeppoTakalo]

Please review:
@kjbracey-arm
@ARMmbed/team-ublox
@ARMmbed/team-realtek

[cmonr]

@SeppoTakalo Wasn't able to add @ARMmbed/team-ublox to the reviewers list for some reason.

[SeppoTakalo]

Its fine if Kevin approve, for Ublox and Realtek this is just for information.

[cmonr]

/morph build

[0xc0170]

license headers add to the new files

[SeppoTakalo]

Added

[mbed-ci]

Build : SUCCESS

Build number : 2386
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7259/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2021
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7259/

[mbed-ci]

Test : FAILURE

Build number : 2174
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7259/2174

[cmonr]

Restarting build since commits after the last command moved the PR's Head sha.
/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2389
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7259/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2026
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7259/

[mbed-ci]

Test : SUCCESS

Build number : 2177
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7259/2177