Rewrite error addition interface

The previous implementation of the error addition interface did not comply
with the invasive testing architecture guidelines. This commit fixes that
by:

- Renaming functions/macros/variables to follow the mbedtls_error_xxx or
  mbedtls_test_hook_xxx convention.

- Making mbedtls_test_hook_error_add a global variable that can be set
  by the testing code.

- Using a static inline function call, as opposed to macro, to keep
  discrepancies between debug and production version to a minimum.

Signed-off-by: Chris Jones <christopher.jones@arm.com>

scripts/data_files/error.fmt

@@ -82,22 +82,6 @@ LOW_LEVEL_CODE_CHECKS
     return( NULL );
 }
 
-#if defined(MBEDTLS_TEST_HOOKS)
-static void (*err_add_hook)( int, int, const char *, int );
-
-void mbedtls_set_err_add_hook( void (*hook)( int, int, const char *, int ) )
-{
-    err_add_hook = hook;
-}
-
-int mbedtls_err_add( int high, int low, const char *file, int line )
-{
-    if( err_add_hook != NULL )
-        (*err_add_hook)( high, low, file, line );
-    return ( high + low );
-}
-#endif /* MBEDTLS_TEST_HOOKS */
-
 void mbedtls_strerror( int ret, char *buf, size_t buflen )
 {
     size_t len;