Bootloader infrastructure and main logic (#404)

* adds bootloader code without a SHA256 implementation

* small fixes and typos

bootloader/memory.x

@@ -0,0 +1,21 @@
+/* Copyright 2021 Google LLC
+ *
+ * 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.
+ */
+
+MEMORY
+{
+  FLASH (rx) : ORIGIN = 0x00000000, LENGTH = 16K
+  RAM (rwx) : ORIGIN = 0x20020000, LENGTH = 128K
+}
+