Add possibility to use external scripts to detect target, flash firmware, reset board and discover serial port for custom boards about mbed-tools HOT 3 OPEN

This looks like a promising approach, using CMake to hook in flashing and debugging functionality ARMmbed/mbed-os#14010

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The approach with CMake provides convent way to upload programs with predefined target name flash-xxx and start debugging with debug-xxx, but:

1. According documentation ("Further Customization" in UploadMethods.md) you need to modify mbed-os/tools/cmake/upload_methods if you want to create new upload method or extend existed one.
2. You need to rebuild project if flash/debug method or its option (gdb port, target id, etc.) is changed.
3. It isn't difficult to write simple bash/python wrapper script that can flash target with openocd, pyocd with existing tools. Additionally many IDE already have integration with OpenOCD/PyOCD.
4. It doesn't solve problems with greentea tests, as this approach doesn't have integration with other mbed tools and doesn't provide way to resolve serial port, that can be a separate device.

So I think that it would be better to have some extension points in the mbed-tools itself (especially due item 4), rather than in the build system.

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I believe this would be very useful to help us to have similar functionality compared to the plugins for the old mbed-os-tools

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