Implemented "$ platformio serialports" command
Allowed special build flags only for "src" files via "SRCBUILD_FLAGS" or global environment "export $PIOSRCBUILD_FLAGS"
Allowed to change default ".pioenvs" directory with global env "export $PIOENVS_DIR"
Added support to override UPLOAD_PORT via cli-option "$ platformio run --upload-port="
Implemented especially for SmartAnthill "$ platformio run -t uploadlazy" target (no dependencies to framework libs, ELF and etc.)