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 .
```
