Make quantum/split_common/serial.[ch] configurable (#4419)
* add temporary compile test shell script
* add 'CONFIG_H += serial_backward_compatibility.h' into common_features.mk:SPLIT_KEYBOARD block
* add quantum/split_common/serial_backward_compatibility.h
SERIAL_PIN_PORT and other PIN define move to serial_backward_compatibility.h
SERIAL_BACKLIT_START move to split_util.h
* quantum/split_common/serial.c change to helix-serial.c style serial configuration
* add temporary file quantum/split_common/split-keyboards-list.txt
* add '#define SOFT_SERIAL_PIN D0' to keyboards/6lit/config.h
* add '#define SOFT_SERIAL_PIN D0' to keyboards/divergetm2/config.h
* add '#define SOFT_SERIAL_PIN D0' to keyboards/ergotravel/rev1/config.h
* add '#define SOFT_SERIAL_PIN D0' to keyboards/foobar/config.h
* add '#define SOFT_SERIAL_PIN D0' to keyboards/handwired/dactyl_manuform/config.h
* add '#define SOFT_SERIAL_PIN D0' to keyboards/handwired/qc60/config.h
* add '//#define SOFT_SERIAL_PIN D0' to keyboards/handwired/xealous/config.h
* add '#define SOFT_SERIAL_PIN D0' to keyboards/iris/rev*/config.h
* add '//#define SOFT_SERIAL_PIN D0' to keyboards/lets_split_eh/config.h
* add '#define SOFT_SERIAL_PIN D0' to keyboards/levinson/rev*/config.h
* add '#define SOFT_SERIAL_PIN D0' to keyboards/miniaxe/config.h
* add '#define SOFT_SERIAL_PIN D0' to keyboards/nyquist/rev?/config.h
* add '#define SOFT_SERIAL_PIN D0' to keyboards/quefrency/rev1/config.h
* add '#define SOFT_SERIAL_PIN D0' to keyboards/qwertyydox/config.h,keyboards/qwertyydox/rev1/config.h
* add '#define SOFT_SERIAL_PIN D0' to keyboards/redox/rev1/config.h
* add '#define SOFT_SERIAL_PIN D0' to keyboards/rorschach/rev1/config.h
* remove '#define SOFT_SERIAL_PIN D0' from quantum/split_common/serial_backward_compatibility.h
* remove temporary file quantum/split_common/split-keyboards-list.txt
* remove temporary compile test shell script
* Revert "remove temporary compile test shell script"
This reverts commit 15b0021b4092127b8d8a21f572642ad3702b46d4.
* update quantum/split_common/compile_split_test.sh for new keyboard test
* add '#define SOFT_SERIAL_PIN D0' to keyboards/diverge3/config.h
* add '#define SOFT_SERIAL_PIN D0' to keyboards/40percentclub/25/config.h
* remove temporary compile test shell script
* update docs/config_options.md, quantum/template/avr/config.h
* fix quantum/template/avr/config.h
* fix typo docs/config_options.md
155e9310ff