fix warnings found in nrf52 build

src/RedirectablePrint.cpp

@@ -38,7 +38,7 @@ size_t RedirectablePrint::vprintf(const char *format, va_list arg)
         va_end(arg);
         return 0;
     };
-    if (len >= printBufLen) {
+    if (len >= (int) printBufLen) {
         delete[] printBuf;
         printBufLen *= 2;
         printBuf = new char[printBufLen];

src/mesh/Channels.cpp

@@ -13,7 +13,7 @@ Channels channels;
 uint8_t xorHash(const uint8_t *p, size_t len)
 {
     uint8_t code = 0;
-    for (int i = 0; i < len; i++)
+    for (size_t i = 0; i < len; i++)
         code ^= p[i];
     return code;
 }

src/plugins/ExternalNotificationPlugin.cpp

@@ -114,7 +114,8 @@ int32_t ExternalNotificationPlugin::runOnce()
 
         return (INT32_MAX);
     }
-
+#else
+    return INT32_MAX;
 #endif
 }
 

src/plugins/SerialPlugin.cpp

@@ -124,7 +124,8 @@ int32_t SerialPlugin::runOnce()
 
         return (INT32_MAX);
     }
-
+#else
+    return INT32_MAX;
 #endif
 }