Fix clang warnings (#4988)

* Fix clang warnings

- signed char on AVR vs unsigned char on ARM
- use references instead of points in calculation (same code size)
- Ignore packed address warning for mavlink includes
- Turn error on packed addresses to warning for Horus (Cannot fix that without major refactoring, might even need eeprom change)

* Revert fixes for clang on AVR as the old AVR gcc does not like them

radio/src/opentx.cpp

@@ -2483,7 +2483,18 @@ void opentxInit(OPENTX_INIT_ARGS)
     // g_model.topbarData is still zero here (because it was not yet read from SDCARD),
     // but we only remember the pointer to in in constructor.
     // The storageReadAll() needs topbar object, so it must be created here
+#if __clang__
+// clang does not like this at all, turn into a warning so that -Werror does not stop here
+// taking address of packed member 'topbarData' of class or structure 'ModelData' may result in an unaligned pointer value [-Werror,-Waddress-of-packed-member]
+#pragma clang diagnostic push
+#pragma clang diagnostic warning "-Waddress-of-packed-member"
+#endif
     topbar = new Topbar(&g_model.topbarData);
+#if __clang__
+// Restore warnings
+#pragma clang diagnostic pop
+#endif
+
     // lua widget state must also be prepared before the call to storageReadAll()
     LUA_INIT_THEMES_AND_WIDGETS();
   }