* add temporary compile test shell script
* Extended support of SKIP_VERSION to make invariant compile results during testing.
* build_keyboard.mk, tmk_core/rules.mk: add LIB_SRC, QUANTUM_LIB_SRC support
Support compiled object enclosed in library.
e.g.
```
LIB_SRC += xxxx.c
xxxx.c --> xxxx.o ---> xxxx.a
```
* remove 'ifdef/ifndef USE_I2C' from quantum/split_common/{i2c|serial}.c
* add SKIP_DEBUG_INFO into tmk_core/rules.mk
When SKIP_DEBUG_INFO=yes is specified, do not use the -g option at compile time.
* tmk_core/rules.mk: Library object need -fno-lto
* add SKIP_DEBUG_INFO=yes
* remove temporary compile test shell script
* add '#define SOFT_SERIAL_PIN D0' to keyboards/lets_split/rev?/config.h
* quantum/split_common/serial.c: Changed not to use USE_I2C.
Takeshi ISHII