boards generator updates:
* simplified mapping description: only flash and spiffs sizes are needed
* some ldscripts are renamed due to autogenerated names
* +2M/0, +2M/512K spiffs, +4M0
* reduce length of hidden strings in boards.txt (#5100, https://github.com/arduino/arduino-builder/issues/284)
* give more details in ldscripts (address, size, +rf-cal, +sdk-wifi-settings)