Rename ext/mbedtls to ext/mbedtls-asn1

The ext/mbedtls directory has a copy of the ASN.1 parser from the mbed
TLS library.  To allow a future change to bring the entire mbed TLS
library in as a submodule under ext, rename this to mbedtls-asn1 to make
it clear this is only a subset.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/ext/mbedtls-asn1/pkg.yml b/ext/mbedtls-asn1/pkg.yml
new file mode 100644
index 0000000..148f9c7
--- /dev/null
+++ b/ext/mbedtls-asn1/pkg.yml
@@ -0,0 +1,24 @@
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#  http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+#
+
+pkg.name: ext/mbedtls-asn1
+pkg.description: "MCUboot's bundled mbed-tls ASN1 parser"
+pkg.author: "Apache Mynewt <dev@mynewt.apache.org>"
+pkg.homepage: "http://mynewt.apache.org/"
+pkg.keywords: