Address review findings on WiFi companion support

- scope the serial config CLI to RP2040; it was exposing the rescue CLI
  (cat/rm/erase) on every ESP32 WiFi build, which gates it behind a
  physical long-press
- bound and space out RP2040 rejoins: the core's join busy-waits, so cap
  it at 5s and retry every 30s instead of every 10s
- stamp the reconnect timer in setup(), so the first loop() doesn't tear
  down an association that is still finishing DHCP
- treat stored credentials as a pair, and pass NULL (not "") for an open
  network
- teach build_as_lib.py where SerialWifiInterface moved

examples/companion_radio/MyMesh.cpp

@@ -2416,8 +2416,9 @@ void MyMesh::loop() {
     checkCLIRescueCmd();
   } else {
     checkSerialInterface();
-#if defined(WIFI_SSID) && !defined(ENABLE_USB_INTERFACE)
-    // headless WiFi build: USB serial isn't a companion transport, so use it for config
+#if defined(WIFI_SSID) && defined(RP2040_PLATFORM) && !defined(ENABLE_USB_INTERFACE)
+    // RP2040 WiFi builds are headless and have no way into the rescue CLI (that needs a
+    // display + long-press), so serve config commands on the otherwise unused USB serial
     checkCLIRescueCmd();
 #endif
   }