Add Raspberry Pi Pico port (#1232)

* Add Raspberry Pi Pico port
- Adds BACnet/IP with abstracted network layer
- Adds BACnet MS/TP support using RS485/UART
- Includes README with examples

* Add Pico 2 MS/TP demo, CI, and docs

ports/pico/dlenv.h

@@ -0,0 +1,42 @@
+/**************************************************************************
+ *
+ * Copyright (C) 2025 Testimony Adams <adamstestimony@gmail.com>
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later WITH GCC-exception-2.0
+ *
+ *********************************************************************/
+
+#ifndef DLENV_H
+#define DLENV_H
+#include <stdint.h>
+#include <stdbool.h>
+
+#include "bacnet/bacdef.h"
+#include "bacnet/datalink/dlenv.h"
+#include "bacnet/datalink/dlmstp.h"
+#include "bacnet/datalink/mstp.h"
+#include "bacnet/basic/sys/mstimer.h"
+
+#include "rs485.h"
+
+#define BACNET_MSTP_MAX_MASTER 5
+#define BACNET_MSTP_MAX_INFO_FRAMES 1
+
+#ifdef __cplusplus
+extern "C" {
+#endif /* __cplusplus */
+
+bool pico_dlenv_init(uint8_t mac_address);
+void dlenv_maintenance_timer(uint16_t seconds);
+void dlenv_cleanup(void);
+
+// TO BE IMPROVED
+extern volatile bool g_mstp_have_token;
+extern volatile uint8_t g_last_frame_type;
+extern volatile uint8_t g_last_src;
+extern volatile uint8_t g_last_dst;
+
+#ifdef __cplusplus
+}
+#endif /* __cplusplus */
+#endif