Production/Alternate test header

@maclobdell

We have decided to use a PCIE-x1 edge connector as our standard test/debug header.
We've broken out SWD, multiple serial ports, QSPI, USB, power, etc.

Not having the time nor intimate knowledge of testing for Mbed OS, we have not put a great amount of thought into which signals and where to put them on the header

Considering what seems to be a lack of formal specification for the Arduino header, would it be beneficial to specify an alternative test/debug/expansion header for Mbed OS boards?

As of this moment we will probably be adding an Arduino header to our CI/test/debug board in order to support the current Mbed ci-test-shield over our PCIE-x1 connector.

The issue mentions a PCIe-x1 connector with SWD, serial, QSPI, USB, and power signals, alongside the Arduino header and Mbed CI test shield; begin by reviewing those existing header requirements. Done would require an agreed alternative test/debug/expansion-header specification, but no file or test is identified in the issue.

Written by the indexing model from the issue text.