ARM mbedtls: prevent compilation of self_test functions

They were discarded by the linker GC as they weren't used so far, but better to make it explicit.

common/mbedtls/config.h

@@ -1447,7 +1447,9 @@
  *
  * Enable the checkup functions (*_self_test).
  */
+#ifndef ON_DEVICE
 #define MBEDTLS_SELF_TEST
+#endif // ON_DEVICE
 
 /**
  * \def MBEDTLS_SHA256_SMALLER