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.

quantum/split_common/i2c.c

@@ -7,8 +7,6 @@
 #include "i2c.h"
 #include "split_flags.h"
 
-#if defined(USE_I2C) || defined(EH)
-
 // Limits the amount of we wait for any one i2c transaction.
 // Since were running SCL line 100kHz (=> 10μs/bit), and each transactions is
 // 9 bits, a single transaction will take around 90μs to complete.
@@ -184,4 +182,3 @@ ISR(TWI_vect) {
   // Reset everything, so we are ready for the next TWI interrupt
   TWCR |= (1<<TWIE) | (1<<TWINT) | (ack<<TWEA) | (1<<TWEN);
 }
-#endif

quantum/split_common/serial.c

@@ -12,18 +12,14 @@
 #include <stdbool.h>
 #include "serial.h"
 
-#ifndef USE_I2C
-
-#ifndef SOFT_SERIAL_PIN
-  #error quantum/split_common/serial.c need SOFT_SERIAL_PIN define
-#endif
+#ifdef SOFT_SERIAL_PIN
 
 #ifdef __AVR_ATmega32U4__
   // if using ATmega32U4 I2C, can not use PD0 and PD1 in soft serial.
-  #ifdef USE_I2C
-    #if SOFT_SERIAL_PIN == D0 || SOFT_SERIAL_PIN == D1
-      #error Using ATmega32U4 I2C, so can not use PD0, PD1
-    #endif
+  #ifdef USE_AVR_I2C
+     #if SOFT_SERIAL_PIN == D0 || SOFT_SERIAL_PIN == D1
+       #error Using ATmega32U4 I2C, so can not use PD0, PD1
+     #endif
   #endif
 
   #if SOFT_SERIAL_PIN >= D0 && SOFT_SERIAL_PIN <= D3
@@ -278,4 +274,4 @@ int serial_update_buffers(void) {
   return 0;
 }
 
-#endif
+#endif /* SOFT_SERIAL_PIN */