Declare flash_default_config() in rtl837x_common.h

boot.c calls it from BANK3 and only rtlplayground.c had a prototype, so
the compiler saw an implicit declaration. The call itself was fine, the
function lives in the common area, but the warning was real. Move the
prototype into the shared header.

Reported by vDorst.

rtlplayground.c

@@ -35,7 +35,6 @@ extern __xdata uint32_t flash_size;
 extern __xdata uint16_t crc_value;
 __xdata struct machine_runtime machine_detected;
 void crc16_bank1(__xdata uint8_t *v) __naked;
-void flash_default_config(void);
 
 // See setup_serial_timer1() for valid baudrate settings!
 #define SERIAL_BAUD_RATE 115200