Define feature test macros in common.h

This enables necessary functions once for all.

As a consequence, define common.h before any other header.

app/src/util/buffer_util.h

@@ -1,11 +1,11 @@
 #ifndef BUFFER_UTIL_H
 #define BUFFER_UTIL_H
 
+#include "common.h"
+
 #include <stdbool.h>
 #include <stdint.h>
 
-#include "common.h"
-
 static inline void
 buffer_write16be(uint8_t *buf, uint16_t value) {
     buf[0] = value >> 8;