Add support for all SHA modes in cert_write

Add support for `MBEDTLS_SHA_224` and `MBEDTLS_SHA_384` in
`cert_write`, to support generating such certificates in
`tests/data_files/Makefile`.
diff --git a/programs/x509/cert_write.c b/programs/x509/cert_write.c
index c3e89be..d36d73c 100644
--- a/programs/x509/cert_write.c
+++ b/programs/x509/cert_write.c
@@ -357,8 +357,12 @@
         {
             if( strcmp( q, "SHA1" ) == 0 )
                 opt.md = MBEDTLS_MD_SHA1;
+            else if( strcmp( q, "SHA224" ) == 0 )
+                opt.md = MBEDTLS_MD_SHA224;
             else if( strcmp( q, "SHA256" ) == 0 )
                 opt.md = MBEDTLS_MD_SHA256;
+            else if( strcmp( q, "SHA384" ) == 0 )
+                opt.md = MBEDTLS_MD_SHA384;
             else if( strcmp( q, "SHA512" ) == 0 )
                 opt.md = MBEDTLS_MD_SHA512;
             else if( strcmp( q, "MD2" ) == 0 )