Refactor: Fix issues found by cppcheck

Resolve several issues found by cppcheck:

    [msl_verify.hpp:28]: (style) Class 'MinimumShipLevel' has a constructor with 1 argument that is not explicit.
    [ubi/watch.hpp:21]: (warning) Assignment of function parameter has no effect outside the function. Did you forget dereferencing it?
    [item_updater_main.cpp:22] -> [item_updater_main.cpp:49]: (style) Local variable rc shadows outer symbol
    [serialize.cpp:19]: (performance) Function parameter 'versionId' should be passed by const reference.
    [ubi/watch.hpp:43]: (style) Struct 'CustomFd' has a constructor with 1 argument that is not explicit.
    [serialize.cpp:52]: (performance) Function parameter 'versionId' should be passed by const reference.
    [serialize.cpp:116]: (performance) Function parameter 'versionId' should be passed by const reference.
    [activation.cpp:115] -> [activation.cpp:152]: (style) Local variable mapperResponseMsg shadows outer symbol
    [version.hpp:114]: (performance) Variable 'eraseCallback' is assigned in constructor body. Consider performing initialization in initialization list.
    [image_verify.hpp:53]: (style) Struct 'CustomFd' has a constructor with 1 argument that is not explicit.
    [ubi/item_updater_ubi.cpp:192]: (performance) Function parameter 'versionId' should be passed by const reference.
    [ubi/item_updater_ubi.cpp:203]: (performance) Function parameter 'versionId' should be passed by const reference.

Tested: Verify the code compiles and cppcheck does not report the above
        issues.

Change-Id: I096392a2a7a283fe198c9c29185125e61295e10f
Signed-off-by: Lei YU <mine260309@gmail.com>
diff --git a/image_verify.hpp b/image_verify.hpp
index 768182e..31b5742 100644
--- a/image_verify.hpp
+++ b/image_verify.hpp
@@ -50,7 +50,7 @@
      *
      *  @param[in] fd - File descriptor
      */
-    CustomFd(int fd) : fd(fd)
+    explicit CustomFd(int fd) : fd(fd)
     {
     }