Add boot image signing keys.

Change-Id: I31fdeedcb845242d57608543c9e0cedb6ebd5ca2
diff --git a/excelsior.xml b/excelsior.xml
index e6b9ba3..cbe4c74 100644
--- a/excelsior.xml
+++ b/excelsior.xml
@@ -57,6 +57,8 @@
       name="excelsior-mendel/packages/linux-mtk" />
     <project remote="excelsior" path="packages/partition-table"
       name="excelsior-mendel/packages/partition-table" />
+    <project remote="excelsior" path="packages/boot-keys"
+      name="excelsior-mendel/packages/boot-keys" />
     <!-- END mtk packages repos -->
 
     <!-- END Excelsior specfic repos -->