sha: make SHA-224 independent from SHA-256

Using proper configuration options (i.e. MBEDTLS_SHA224_C and
MBEDTLS_SHA256_C) it is now possible to build SHA224 and SHA256
independently from each other.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>
diff --git a/include/mbedtls/sha256.h b/include/mbedtls/sha256.h
index 0cbbac1..979ad29 100644
--- a/include/mbedtls/sha256.h
+++ b/include/mbedtls/sha256.h
@@ -179,7 +179,15 @@
 #if defined(MBEDTLS_SELF_TEST)
 
 /**
- * \brief          The SHA-224 and SHA-256 checkup routine.
+ * \brief          The SHA-224 checkup routine.
+ *
+ * \return         \c 0 on success.
+ * \return         \c 1 on failure.
+ */
+int mbedtls_sha224_self_test( int verbose );
+
+/**
+ * \brief          The SHA-256 checkup routine.
  *
  * \return         \c 0 on success.
  * \return         \c 1 on failure.