Refactor quantum/split_common/i2c.c, quantum/split_common/serial.c (#4522)

* 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.

tmk_core/common/command.c

@@ -181,7 +181,11 @@ static void print_version(void)
     print("VID: " STR(VENDOR_ID) "(" STR(MANUFACTURER) ") "
           "PID: " STR(PRODUCT_ID) "(" STR(PRODUCT) ") "
           "VER: " STR(DEVICE_VER) "\n");
+#ifdef SKIP_VERSION
+    print("BUILD:  (" __DATE__ ")\n");
+#else
     print("BUILD: " STR(QMK_VERSION) " (" __TIME__ " " __DATE__ ")\n");
+#endif
 
     /* build options */
     print("OPTIONS:"