Fixed linker error on release build

Author: blckmn

src/main/drivers/drv_uart.c

@@ -3,6 +3,7 @@
 #include "stm32f0xx_ll_dma.h"
 #include "stm32f0xx_ll_gpio.h"
 
+#ifdef DEBUG
 int _write (int fd, char *ptr, int len)
 {
  UNUSED(fd);
@@ -11,6 +12,7 @@ int _write (int fd, char *ptr, int len)
  }
  return len;
 }
+#endif
 
 static serialPort_t serialPort;
 

src/main/drivers/drv_uart.h

@@ -41,4 +41,7 @@ void serialWrite(char ch);
 void serialPrint(const char *str);
 bool serialAvailable(void);
 char serialRead(void);
-int _write (int fd, char *ptr, int len);
+
+#ifdef DEBUG
+int _write (int fd, char *ptr, int len);
+#endif