Expose build-properties flag

[ianfixes]

via arduino/arduino-cli#1044

./arduino-cli compile -b arduino:avr:uno --build-property="build.extra_flags=\"-DMY_DEFINE=\"hello world\"\"" /home/user/Arduino/MySketch

This should probably appear in the config as a new entry build_properties

compile:
  build_properties:
    - "build.extra_flags=\"-DMY_DEFINE=\"hello world\"\""
    - "build.extra_flags=-DPIN=2 \"-DMY_DEFINE=\"hello world\"\""
    - build.extra_flags=-DPIN=2
    - "compiler.cpp.extra_flags=\"-DMY_DEFINE=\"hello world\"\""