[radio-spinel] add log on RCP timeout error during init

This commit adds new logs to `HandleRcpTimeout()` when an RCP timeout
occurs during initialization. This error is often incorrectly
reported as a bug, but it is usually caused by an incorrect
configuration or a bad RCP image. The new logs aim to provide
information and hints to the user about the cause of the error.

src/lib/spinel/radio_spinel_impl.hpp

@@ -2298,6 +2298,14 @@ void RadioSpinel<InterfaceType, ProcessContextType>::HandleRcpTimeout(void)
 #if OPENTHREAD_SPINEL_CONFIG_RCP_RESTORATION_MAX_COUNT > 0
     mRcpFailed = true;
 #else
+    if (!mIsReady)
+    {
+        otLogCritPlat("Failed to communicate with RCP - no response from RCP during initialization");
+        otLogCritPlat("This is not a bug and typically due a config error (wrong URL parameters) or bad RCP image:");
+        otLogCritPlat("- Make sure RCP is running the correct firmware");
+        otLogCritPlat("- Double check the config parameters passed as `RadioURL` input");
+    }
+
     DieNow(OT_EXIT_RADIO_SPINEL_NO_RESPONSE);
 #endif
 }