Add random delay to enforce_volatile_reads

Add a random delay to mbedtls_platform_enforce_volatile_reads() as a
countermeasure to fault injection attacks.

include/mbedtls/platform_util.h

@@ -260,9 +260,12 @@ uint32_t mbedtls_platform_random_in_range( size_t num );
 int mbedtls_platform_random_delay( size_t num );
 
 /**
- * \brief       This function does nothing, but can be inserted between
- *              successive reads to a volatile local variable to prevent
- *              compilers from optimizing them away.
+ * \brief       This function can be inserted between successive reads to a
+ *              volatile local variable to prevent compilers from optimizing
+ *              them away. In addition, this function will spent a small random
+ *              time in a busy loop as a counter-measure to fault injection
+ *              attack.
+ *
  */
 void mbedtls_platform_enforce_volatile_reads( void );