Add suppport for MD2 to CSR and CRT writing example programs

The example programs programs/x509/cert_req and programs/x509/cert_write
(demonstrating the use of X.509 CSR and CRT writing functionality)
previously didn't support MD2 signatures.

For testing purposes, this commit adds support for MD2 to cert_req,
and support for MD2 and MD4 to cert_write.
diff --git a/programs/x509/cert_req.c b/programs/x509/cert_req.c
index d25ad4c..b2052ec 100644
--- a/programs/x509/cert_req.c
+++ b/programs/x509/cert_req.c
@@ -102,7 +102,7 @@
     "                          Add NsCertType even if it is empty\n"    \
     "    md=%%s               default: SHA256\n"       \
     "                          possible values:\n"     \
-    "                          MD4, MD5, SHA1\n"       \
+    "                          MD2, MD4, MD5, SHA1\n"  \
     "                          SHA224, SHA256\n"       \
     "                          SHA384, SHA512\n"       \
     "\n"
@@ -252,6 +252,13 @@
             }
             else
 #endif /* MBEDTLS_MD5_C */
+#if defined(MBEDTLS_MD2_C)
+            if( strcmp( q, "MD2" ) == 0 )
+            {
+                opt.md_alg = MBEDTLS_MD_MD2;
+            }
+            else
+#endif /* MBEDTLS_MD2_C */
 #if defined(MBEDTLS_SHA1_C)
             if( strcmp( q, "SHA1" ) == 0 )
             {