Merge 3bbd43cc1b into 760ab981d0

Two issues were resolved with KISS interface beacon transmissions:

1. Fixed frame length handling to meet minimum length requirements (15 bytes) for
   TNCs like Direwolf. Previously, raw beacon data was being sent directly,
   causing frame length errors:

```
<<< Data frame from KISS client application, channel 0, total length = 11
  000:  c0 00 44 4e 39 54 54 2d 31 35 c0                 ..FOOBAR-15.
Frame length 8 not in allowable range of 15 to 2123.
ERROR - Invalid KISS data frame from client app.
```

2. Added standard 0xAA 0xAA sync pattern to improve radio modem synchronization
   and compatibility with packet radio protocols like FX.25 and IL2P.

This ensures:
- Proper frame length for TNC compatibility
- Better bit synchronization with standard sync pattern
- Improved reception reliability
- Compatible with both KISS TNCs and packet radio protocols

```
<<< Data frame from KISS client application, channel 0, total length = 18
  000:  c0 00 aa aa 44 4e 39 54 54 2d 31 35 00 00 00 00  ....FOOBAR-15....
  010:  00 c0                                            ..
[0L 21:25:43 UTC] (Not AX.25)<0xaa><0xaa>FOOBAR-15<0x00><0x00><0x00><0x00><0x00>
IL2P frame, max_fec = 1, 49 encoded bytes total
  000:  f1 5e 48 87 b8 59 b7 a1 cc 24 57 d6 0b 36 7c f1  .^H..Y...$W..6|.
  010:  7c d5 af 85 db 12 99 80 9d b8 6b bb 6b 05 df 1e  |.........k.k...
  020:  69 0e d8 69 05 f6 ff c4 24 fa 58 f7 e7 f2 a4 df  i..i....$.X.....
  030:  a1                                               .
```

RNS/Interfaces/KISSInterface.py

@@ -338,7 +338,14 @@ class KISSInterface(Interface):
                             if time.time() > self.first_tx + self.beacon_i:
                                 RNS.log("Interface "+str(self)+" is transmitting beacon data: "+str(self.beacon_d.decode("utf-8")), RNS.LOG_DEBUG)
                                 self.first_tx = None
-                                self.processOutgoing(self.beacon_d)
+                                
+                                # Using standard HDLC flag or sync bytes
+                                frame = bytearray([0xAA, 0xAA])  # Standard sync pattern
+                                frame.extend(self.beacon_d)
+                                while len(frame) < 15:
+                                    frame.append(0x00)
+                                    
+                                self.processOutgoing(bytes(frame))
 
         except Exception as e:
             self.online = False