add capability to switch out the public default key for OEMs (#216)

deviceonly.options

@@ -10,5 +10,5 @@
 *ChannelFile.channels max_count:8
 
 *OEMStore.oem_text max_size:40
-
-*OEMStore.oem_icon_bits max_size:2048
+*OEMStore.oem_icon_bits max_size:2048
+*OEMStore.oem_aes_key max_size:32

deviceonly.proto

@@ -134,4 +134,9 @@ message OEMStore {
    * Use this font for the OEM text.
    */
   string oem_text = 5;
+
+  /*
+   * The default device encryption key, 16 or 32 byte
+   */
+  bytes oem_aes_key = 6;
 }