Remove dependency of ssl_cookie on SHA-224

Cookies are fully opaque so we can change the hash used at any time, it's not
part of the API.

The cookie module handles truncation, so it's simpler to always use SHA-256
rather than check if SHA-224 is available.
diff --git a/library/ssl_cookie.c b/library/ssl_cookie.c
index 56e9bdd..6f0be8a 100644
--- a/library/ssl_cookie.c
+++ b/library/ssl_cookie.c
@@ -50,7 +50,7 @@
  * with max 32 bytes of cookie for DTLS 1.0
  */
 #if defined(MBEDTLS_SHA256_C)
-#define COOKIE_MD           MBEDTLS_MD_SHA224
+#define COOKIE_MD           MBEDTLS_MD_SHA256
 #define COOKIE_MD_OUTLEN    32
 #define COOKIE_HMAC_LEN     28
 #elif defined(MBEDTLS_SHA512_C)