net: keep network packets out of the console (fixes shell garbage on netboot)

The link port is both the console-input fallback and the network. At the shell
prompt the host's multicast (mDNS/LLMNR/IGMP) was fed to the serial and KGetc
read those packet bytes as console input -> garbage in the shell, OSK unusable.

Two-part fix:
- KGetc now skips SLIP frames (0xC0-delimited) on the serial console path, so
  inbound packets never surface as console input. Plain injected bytes (the
  headless tunbridge command path) still pass through. New wConInSkip flag.
- netboot/tunbridge only forward IP packets destined to 10.0.0.2 (drop the
  multicast noise at the bridge).

Verified: GB sits at a clean "/#" prompt while all packets (noise included) are
forwarded; being-pinged (3/3) and wget still work. On the emulator, the OSK is
SELECT=space, START=enter, A=z, B=x, d-pad=WASD/arrows.

1 files changed, 2 insertions, 0 deletions

diff --git a/tools/tunbridge.py b/tools/tunbridge.py
index b462c1b..f5e391c 100644
--- a/tools/tunbridge.py
+++ b/tools/tunbridge.py
@@ -68,6 +68,8 @@ def main():
         while True:
             pkt = os.read(tun, 2048)
             if not pkt: return
+            if len(pkt) < 20 or pkt[16:20] != b"\x0a\x00\x00\x02":
+                continue                   # only unicast to 10.0.0.2 (drop mcast noise)
             try: p.stdin.write(slip_encode(pkt)); p.stdin.flush()
             except Exception: return