Add a new self test to entropy module

The self test is a quick way to check at startup whether the entropy
sources are functioning correctly. The self test only polls 8 bytes
from the default entropy source and performs the following checks:

- The bytes are not all 0x00 or 0xFF.
- The hardware does not return an error when polled.
- The entropy does not provide data in a patter. Only check pattern
  at byte, word and long word sizes.

include/mbedtls/entropy.h

@@ -258,6 +258,15 @@ int mbedtls_entropy_update_seed_file( mbedtls_entropy_context *ctx, const char *
  * \return         0 if successful, or 1 if a test failed
  */
 int mbedtls_entropy_self_test( int verbose );
+
+#if !defined(MBEDTLS_TEST_NULL_ENTROPY) && defined(MBEDTLS_ENTROPY_HARDWARE_ALT)
+/**
+ * \brief          Checkup routine
+ *
+ * \return         0 if successful, or 1 if a test failed
+ */
+int mbedtls_entropy_self_test_bias( int verbose );
+#endif /* !MBEDTLS_TEST_NULL_ENTROPY && MBEDTLS_ENTROPY_HARDWARE_ALT */
 #endif /* MBEDTLS_SELF_TEST */
 
 #ifdef __cplusplus