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/image_verify.hpp b/image_verify.hpp
index 31b5742..0aff4b8 100644
--- a/image_verify.hpp
+++ b/image_verify.hpp
@@ -31,9 +31,6 @@
 using EVP_MD_CTX_Ptr =
     std::unique_ptr<EVP_MD_CTX, decltype(&::EVP_MD_CTX_free)>;
 
-// PNOR flash image file name.
-constexpr auto squashFSImage = "pnor.xz.squashfs";
-
 /** @struct CustomFd
  *
  *  RAII wrapper for file descriptor.
@@ -133,7 +130,9 @@
      * @param[in]  signedConfPath - Path of public key
      *                              hash function files
      */
-    Signature(const fs::path& imageDirPath, const fs::path& signedConfPath);
+    explicit Signature(const fs::path& imageDirPath,
+                       const std::string& pnorFileName,
+                       const fs::path& signedConfPath);
 
     /**
      * @brief Image signature verification function.
@@ -205,6 +204,9 @@
     /** @brief Directory where software images are placed*/
     fs::path imageDirPath;
 
+    /** @brief The PNOR file name in imageDirPath */
+    std::string pnorFileName;
+
     /** @brief Path of public key and hash function files */
     fs::path signedConfPath;