* Add provisional Helix implementation to test the quantum/split_common.
* copy keyboards/helix/serial.[ch] to quantum/split_common/
* Make serial.c a pure driver.
Remove buffer name and buffer size from serial.c. They should be placed in the caller(matrix.c, split_utils.c).
* remove quantum/split_common/serial_backward_compatibility.h
* Changed array serial_master_buffer to structure serial_m2s_buffer.
* Changed array serial_slave_buffer to structure serial_s2m_buffer.
* Change keyboards/miniaxe/matrix.c
I also made changes to quantum/split_comon/matrix.c to keyboards/miniaxe/matrix.c.
Note: I contacted @ka2hiro, creator of miniaxe, and I got permission to change keyboards/miniaxe/matrix.c.
* update history comment in quantum/split_common/serial.c
* Revert "Add provisional Helix implementation to test the quantum/split_common."
This reverts commit 168c82ef82c88e79979d9796bab9cc819cc2f685.
* fix keyboards/miniaxe/matrix.c, quantum/split_common/matrix.c
avr-gcc 4.9.[23] report error.
avr-gcc 5.4.0, avr-gcc 7.3.0 pass.
It is funny.
* update comment quantum/split_common/serial.c
* Reserve RGBLIGHT_SPLIT macro in quantum/split_common
