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/storage/sdcard_raw.cpp

@@ -32,7 +32,7 @@ const char * writeFile(const char * filename, const uint8_t * data, uint16_t siz
   TRACE("writeFile(%s)", filename);
   
   FIL file;
-  char buf[8];
+  unsigned char buf[8];
   UINT written;
 
   FRESULT result = f_open(&file, filename, FA_CREATE_ALWAYS | FA_WRITE);