Obey timeout value on SerialUART.read/.peek (#211)

Fixes a hang when reading from the Serial UART ports because before the
core would pause indefinitely for the next character.

Now, wait up to Serial.setTimeout() milliseconds and if it times out
return -1 to the app.

Fixes #210

cores/rp2040/SerialUART.cpp

@@ -115,7 +115,11 @@ int SerialUART::peek() {
     if (_peek >= 0) {
         return _peek;
     }
-    _peek = uart_getc(_uart);
+    if (uart_is_readable_within_us(_uart, _timeout * 1000)) {
+        _peek = uart_getc(_uart);
+    } else {
+        _peek = -1; // Timeout
+    }
     return _peek;
 }
 
@@ -129,7 +133,11 @@ int SerialUART::read() {
         _peek = -1;
         return ret;
     }
-    return uart_getc(_uart);
+    if (uart_is_readable_within_us(_uart, _timeout * 1000)) {
+        return uart_getc(_uart);
+    } else {
+        return -1; // Timeout
+    }
 }
 
 int SerialUART::available() {