AP_Scripting: introduce serial device simulation support

Allows a script to simulate a device attached via any serial protocol.
The script can read and write data and have it handled according to the
protocol as if exchanged over a serial port. The script can then do
protocol translation, data filtering and validation,
hardware-in-the-loop simulation, experimentation, etc., especially in
combination with the scripting protocol which lets the script itself
handle an attached device and so interpose any communication.

libraries/AP_Scripting/AP_Scripting_SerialAccess.cpp

@@ -1,15 +1,27 @@
 /*
-  generic object to allow a script to use a serial stream
+  generic object to allow a script to use a serial driver stream from both
+  driver and device perspectives
  */
 
 #include "AP_Scripting_config.h"
+#include "AP_Scripting.h"
 #include "AP_Scripting_SerialAccess.h"
 
 #if AP_SCRIPTING_ENABLED
 
+#if AP_SCRIPTING_SERIALDEVICE_ENABLED
+#define check_is_device_port() (is_device_port)
+#define ON_DEVICE_PORT(func, ...) (((AP_Scripting_SerialDevice::Port*)stream)->device_##func (__VA_ARGS__))
+#else
+#define check_is_device_port() (false)
+#define ON_DEVICE_PORT(...) (0) // not executed
+#endif
+
 void AP_Scripting_SerialAccess::begin(uint32_t baud)
 {
-    stream->begin(baud);
+    if (!check_is_device_port()) {
+        stream->begin(baud);
+    }
 }
 
 size_t AP_Scripting_SerialAccess::write(uint8_t c)
@@ -19,7 +31,10 @@ size_t AP_Scripting_SerialAccess::write(uint8_t c)
 
 size_t AP_Scripting_SerialAccess::write(const uint8_t *buffer, size_t size)
 {
-    return stream->write(buffer, size);
+    if (!check_is_device_port()) {
+        return stream->write(buffer, size);
+    }
+    return ON_DEVICE_PORT(write, buffer, size);
 }
 
 int16_t AP_Scripting_SerialAccess::read(void)
@@ -33,17 +48,25 @@ int16_t AP_Scripting_SerialAccess::read(void)
 
 ssize_t AP_Scripting_SerialAccess::read(uint8_t* buffer, uint16_t count)
 {
-    return stream->read(buffer, count);
+    if (!check_is_device_port()) {
+        return stream->read(buffer, count);
+    }
+    return ON_DEVICE_PORT(read, buffer, count);
 }
 
 uint32_t AP_Scripting_SerialAccess::available(void)
 {
-    return stream->available();
+    if (!check_is_device_port()) {
+        return stream->available();
+    }
+    return ON_DEVICE_PORT(available);
 }
 
 void AP_Scripting_SerialAccess::set_flow_control(enum AP_HAL::UARTDriver::flow_control fcs)
 {
-    stream->set_flow_control(fcs);
+    if (!check_is_device_port()) {
+        stream->set_flow_control(fcs);
+    }
 }
 
 #endif // AP_SCRIPTING_ENABLED