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.

programs/test/selftest.c

@@ -375,6 +375,12 @@ int main( int argc, char *argv[] )
     {
         suites_failed++;
     }
+#if !defined(MBEDTLS_TEST_NULL_ENTROPY) && defined(MBEDTLS_ENTROPY_HARDWARE_ALT)
+    if( mbedtls_entropy_self_test_bias( v ) != 0)
+    {
+        suites_failed++;
+    }
+#endif
     suites_tested++;
 #endif