NRF52: Customize STDIO pins from config system

[marcuschangarm]

This change makes it possible to customize the STDIO pins, which is needed by the Client Lite.

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1343
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6275/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 999
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6275/

[mbed-ci]

Test : SUCCESS

Build number : 1126
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6275/1126

[yogpan01]

@0xc0170 @adbridge Can we please get this PR in mbedOS-5.8-rc2 release ? We need this ASAP for us to start testing client on nRF52

[0xc0170]

@0xc0170 @adbridge Can we please get this PR in mbedOS-5.8-rc2 release ? We need this ASAP for us to start testing client on nRF52

It's not clear how this relates to rc2. It is a bugfix but from description in this PR, looks unrelated to our current ongoing release.

[adbridge]

@yogpan01 Normally this would not be allowed to go into RC2 as it was not found during our OOB for RC1. Introducing new code between release candidates is a bad thing to do and could require a whole lot of testing to be repeated. In this particular case as the likely impact on anything else is very low risk, we will take it. But please be aware that this will not always be the case.