[LICENSE] Add SPDX headers to Makefiles

Makefile

diff --git a/Makefile b/Makefile
index 76a245e..18b66a4 100644
--- a/Makefile
+++ b/Makefile
@@ -1,3 +1,19 @@
+# SPDX-FileCopyrightText: 2020 Efabless Corporation
+#
+# Licensed 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.
+#
+# SPDX-License-Identifier: Apache-2.0
+
 # cannot commit files larger than 100 MB to GitHub
 FILE_SIZE_LIMIT_MB = 100
 LARGE_FILES := $(shell find ./gds -type f -name "*.gds")