AP_Scripting: use generic access userdata for serial ports

Adding another layer instead of just exposing UARTDriver bindings allows
substitution of the different functions for device simulation later.

Also take the opportunity to rework the docs a little.

libraries/AP_Scripting/AP_Scripting_SerialAccess.cpp

@@ -0,0 +1,49 @@
+/*
+  generic object to allow a script to use a serial stream
+ */
+
+#include "AP_Scripting_config.h"
+#include "AP_Scripting_SerialAccess.h"
+
+#if AP_SCRIPTING_ENABLED
+
+void AP_Scripting_SerialAccess::begin(uint32_t baud)
+{
+    stream->begin(baud);
+}
+
+size_t AP_Scripting_SerialAccess::write(uint8_t c)
+{
+    return write(&c, 1);
+}
+
+size_t AP_Scripting_SerialAccess::write(const uint8_t *buffer, size_t size)
+{
+    return stream->write(buffer, size);
+}
+
+int16_t AP_Scripting_SerialAccess::read(void)
+{
+    uint8_t c;
+    if (read(&c, 1) != 1) {
+        return -1;
+    }
+    return c;
+}
+
+ssize_t AP_Scripting_SerialAccess::read(uint8_t* buffer, uint16_t count)
+{
+    return stream->read(buffer, count);
+}
+
+uint32_t AP_Scripting_SerialAccess::available(void)
+{
+    return stream->available();
+}
+
+void AP_Scripting_SerialAccess::set_flow_control(enum AP_HAL::UARTDriver::flow_control fcs)
+{
+    stream->set_flow_control(fcs);
+}
+
+#endif // AP_SCRIPTING_ENABLED