Read additional BuildProperties from optional ./platform.sketch.txt file

[phd]

Read additional BuildProperties from optional ./platform.sketch.txt file.

Allows sketches to provide and maintain their own platform.txt properties for use during the build process.
Similar to platform.local.txt solution, but instead this is sketch based.

Provides a command line option -use-platform-sketch-txt to enable this feature in arduino-builder (disabled by default), for use with Arduino IDE.
See: arduino/Arduino#7821

See also:
arduino/ArduinoCore-avr#21
arduino/ArduinoCore-arc32#588
arduino/ArduinoCore-sam#52
arduino/ArduinoCore-samd#335

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