Use slightly flatter directory structure since some developers did not

like too many folders.
Extracted code from some files into separate files to fit with the new
layout.

src/drivers/serial_common.c

@@ -0,0 +1,45 @@
+#include "board.h"
+
+void serialPrint(serialPort_t *instance, const char *str)
+{
+    uint8_t ch;
+    while ((ch = *(str++)) != 0) {
+        serialWrite(instance, ch);
+    }
+}
+
+uint32_t serialGetBaudRate(serialPort_t *instance)
+{
+    return instance->baudRate;
+}
+
+void serialWrite(serialPort_t *instance, uint8_t ch)
+{
+    instance->vTable->serialWrite(instance, ch);
+}
+
+uint8_t serialTotalBytesWaiting(serialPort_t *instance)
+{
+    return instance->vTable->serialTotalBytesWaiting(instance);
+}
+
+uint8_t serialRead(serialPort_t *instance)
+{
+    return instance->vTable->serialRead(instance);
+}
+
+void serialSetBaudRate(serialPort_t *instance, uint32_t baudRate)
+{
+    instance->vTable->serialSetBaudRate(instance, baudRate);
+}
+
+bool isSerialTransmitBufferEmpty(serialPort_t *instance)
+{
+    return instance->vTable->isSerialTransmitBufferEmpty(instance);
+}
+
+inline void serialSetMode(serialPort_t *instance, portMode_t mode)
+{
+    instance->vTable->setMode(instance, mode);
+}
+