Update serial drivers so they do not include "board.h". It is now clear

what all serial drivers need to compile and what was unnecessarily
included before.

src/drivers/serial_common.c

@@ -1,4 +1,10 @@
-#include "board.h"
+
+#include <stdbool.h>
+#include <stdint.h>
+
+#include "platform.h"
+
+#include "serial_common.h"
 
 void serialPrint(serialPort_t *instance, const char *str)
 {

src/drivers/serial_softserial.c

@@ -1,4 +1,15 @@
-#include "board.h"
+
+#include <stdbool.h>
+#include <stdint.h>
+
+#include "platform.h"
+
+#include "system_common.h"
+#include "gpio_common.h"
+#include "timer_common.h"
+
+#include "serial_common.h"
+#include "serial_softserial.h"
 
 #define SOFT_SERIAL_TIMER_MHZ 3
 #define SOFT_SERIAL_1_TIMER_RX_HARDWARE 4

src/drivers/serial_uart.c

@@ -1,4 +1,16 @@
-#include "board.h"
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#include "platform.h"
+
+#include "system_common.h"
+#include "gpio_common.h"
+
+#include "serial_common.h"
+#include "serial_uart.h"
+
 /*
     DMA UART routines idea lifted from AutoQuad
     Copyright © 2011  Bill Nesbitt