Make quantum/split_common/serial.[ch] configurable (#4419)

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