Add KISS TNC external app for APRS RX/TX over USB serial (#3078)

* Add KISS TNC external app for APRS RX/TX over USB serial

Adds a KISS TNC (Terminal Node Controller) app that bridges APRS
packets between the HackRF radio and a connected PC over USB CDC
serial using the KISS protocol.

Features:
- APRS receive at 144.390 MHz (configurable), decoding packets via
  the existing APRS RX baseband (PAPR.bin)
- Received AX.25 frames forwarded to host as KISS-framed data over
  USB serial
- KISS frames received from host transmitted as AFSK audio using the
  existing AFSK TX baseband (PAFT.bin)
- Clean RX->TX->RX baseband switching with proper timing delays
- USB connection status indicator with periodic refresh
- Compatible with standard KISS TNC clients (Xastir, APRSISCE/32,
  Dire Wolf, etc.)

Firmware changes required to support the external app:
- baseband_api.cpp: mark set_aprs() __attribute__((used)) so it is
  retained by the linker for external app use
- protocols/ax25: add make_frame_from_raw() to build NRZI bitstream
  from raw AX.25 bytes, marked __attribute__((used))
- usb_serial_host_to_device: add kiss_raw_handler hook to route
  incoming USB bytes directly to a registered callback, bypassing
  the shell; marked __attribute__((used))
- baseband/proc_aprsrx: add constructor that auto-configures at
  1200 baud so external apps do not need to call set_aprs() at init

* Address PR review: generic RAII USB handler, fix TX cutoff and buffer overflow

- Replace KISS-specific set_kiss_raw_handler() in shared USB code with a
  generic UsbSerialInputHandler RAII class; any app can now register a
  handler via the ctor and it auto-clears in the dtor — no app-specific
  'if' checks in shared code, no __attribute__((used)) needed
- Remove __attribute__((used)) from set_aprs() — it is referenced directly
  by ui_aprs_rx.cpp so it will never be dead-stripped
- Fix send_kiss_frame() buffer overflow: per-byte bounds check prevents
  2-byte escape sequences from writing past the end of the output buffer
- Replace fillOBuffer(TIME_INFINITE) with chOQWriteTimeout(TIME_IMMEDIATE)
  to drop bytes instead of blocking the event loop
- Remove chThdSleepMilliseconds() calls from start_tx() and finish_tx()
- Fix TX cutoff bug: call start_tx() directly from process_kiss_frame()
  instead of deferring via tx_pending_ flag (deferral caused kiss_idx_
  to be 0 by the time start_tx() fired, producing an empty frame)

* Move USB write into UsbSerialInputHandler class

Add a write() method to UsbSerialInputHandler so all USB I/O
(both input callback and output) is encapsulated in the class.
KISS TNC now uses usb_input_handler_->write() instead of calling
chOQWriteTimeout(&SUSBD1.oqueue, ...) directly, keeping raw USB
details out of app code.

* Address PR review: remove auto-configure from APRSRxProcessor, add timeout param to write()

- Remove default_config from APRSRxProcessor() constructor; baseband should
  start unconfigured and wait for APRSRxConfigureMessage from the app side
  (already sent via baseband::set_aprs() in KISS TNC and APRS RX app)
- Remove stray #include "stdio.h" from proc_aprsrx.cpp
- Add systime_t timeout parameter with default TIME_IMMEDIATE to
  UsbSerialInputHandler::write() so callers can choose blocking behaviour

firmware/application/protocols/ax25.cpp

@@ -131,4 +131,30 @@ void AX25Frame::make_ui_frame(char* const address, const uint8_t control, const
     flush();
 }
 
+// __attribute__((used)) prevents dead-stripping; called only from KISS TNC external app.
+__attribute__((used)) void AX25Frame::make_frame_from_raw(const uint8_t* data, size_t len) {
+    bb_data_ptr = (uint16_t*)shared_memory.bb_data.data;
+    memset(bb_data_ptr, 0, sizeof(shared_memory.bb_data.data));
+    bit_counter = 0;
+    current_bit = 0;
+    current_byte = 0;
+    ones_counter = 0;
+    crc_ccitt.reset();
+
+    add_flag();
+    add_flag();
+    add_flag();
+    add_flag();
+
+    for (size_t i = 0; i < len; i++)
+        add_data(data[i]);
+
+    add_checksum();
+
+    add_flag();
+    add_flag();
+
+    flush();
+}
+
 } /* namespace ax25 */