Replace serial.c of quantum/split_common/ (#4669)

Replace serial.c of quantum/split_common/ (#4669)

* 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