First cut of configurable serial port functionality.

Currently port usage is hard-coded to the default port layout, cli
commands are coming in a future commit.

This decouples all code from the global 'serialPorts' structure which
has been removed.  Any code that needs to use a serial port can use
findOpenSerialPort() and openSerialPort() and maintain it's own
reference to the port.

Ports can switch between functions.  e.g. by default
cli/msp/telemetry/gps
passthrough all use USART1.  Each port maintains it's current function.
see begin/endSerialPortFunction.

There are only certain combinations of serial port functions that are
supported, these are listed in serialPortFunctionScenario_e.

This commit also adds a few 'static' keywords to variables that should
have been.

There a a few other minor fixes and tweaks to various bits of code that
this uncovered too.

src/drivers/serial_uart_stm32f30x.c

@@ -184,6 +184,7 @@ void DMA1_Channel4_IRQHandler(void)
 {
     uartPort_t *s = &uartPort1;
     DMA_ClearITPendingBit(DMA1_IT_TC4);
+    DMA_Cmd(DMA1_Channel4, DISABLE);
     handleUsartTxDma(s);
 }
 
@@ -209,6 +210,11 @@ void usartIrqHandler(uartPort_t *s)
             s->port.rxBufferHead = (s->port.rxBufferHead + 1) % s->port.rxBufferSize;
         }
     }
+
+    if (s->txDMAChannel) {
+        return;
+    }
+
     if (ISR & USART_FLAG_TXE) {
         if (s->port.txBufferTail != s->port.txBufferHead) {
             s->USARTx->TDR = s->port.txBuffer[s->port.txBufferTail];