Static layout: support image verification

Add support of image verification for static layout PNOR code update.

Tested: Verify the PNOR code update succeeds with valid-signed PNOR;
        and fails with invalid-signed PNOR or a PNOR tarball without
        signature.

Change-Id: I1aafeb4e8e07eaa16c170f33f4f21940f7c9c146
Signed-off-by: Lei YU <mine260309@gmail.com>
diff --git a/ubi/activation_ubi.hpp b/ubi/activation_ubi.hpp
index c0ffa01..a6ef436 100644
--- a/ubi/activation_ubi.hpp
+++ b/ubi/activation_ubi.hpp
@@ -9,6 +9,8 @@
 namespace updater
 {
 
+constexpr auto squashFSImage = "pnor.xz.squashfs";
+
 class RedundancyPriorityUbi : public RedundancyPriority
 {
   public: