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_common.h

@@ -1,5 +1,11 @@
 #pragma once
 
+
+typedef enum {
+    SERIAL_NOT_INVERTED = 0,
+    SERIAL_INVERTED
+} serialInversion_e;
+
 typedef enum portMode_t {
     MODE_RX = 1  << 0,
     MODE_TX = 1 << 1,
@@ -12,8 +18,9 @@ typedef void (* serialReceiveCallbackPtr)(uint16_t data);   // used by serial dr
 typedef struct serialPort {
     
     const struct serialPortVTable *vTable;
-    
+    uint8_t identifier;
     portMode_t mode;
+    serialInversion_e inversion;
     uint32_t baudRate;
 
     uint32_t rxBufferSize;