Add a concept of entropy source strength.

The main goal is, we want and error if cycle counter is the only source.
diff --git a/include/mbedtls/error.h b/include/mbedtls/error.h
index 9244249..4f43018 100644
--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -61,9 +61,8 @@
  * PADLOCK   1  0x0030-0x0030
  * DES       1  0x0032-0x0032
  * CTR_DBRG  4  0x0034-0x003A
- * ENTROPY   3  0x003C-0x0040
+ * ENTROPY   3  0x003C-0x0040   0x003D-0x003F
  * NET       9  0x0042-0x0052   0x0043-0x0043
- * ENTROPY   1  0x0058-0x0058
  * ASN1      7  0x0060-0x006C
  * PBKDF2    1  0x007C-0x007C
  * HMAC_DRBG 4  0x0003-0x0009