config.h.in: use const variables instead of macros

Most style guides try to avoid preprocessor macros, especially the use
case here: const objects. This change replaced them with const
variables. Their names are also changed according to the OpenBMC style
guide.

Reference:
https://google.github.io/styleguide/cppguide.html#Preprocessor_Macros

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I0786c7c83f3a0d892c14f1cb813d0aa16d627b3e
diff --git a/csr.cpp b/csr.cpp
index 1ea3991..677b48b 100644
--- a/csr.cpp
+++ b/csr.cpp
@@ -37,7 +37,7 @@
         elog<InternalFailure>();
     }
     fs::path csrFilePath = certInstallPath;
-    csrFilePath = csrFilePath.parent_path() / CSR_FILE_NAME;
+    csrFilePath = csrFilePath.parent_path() / defaultCSRFileName;
     if (!fs::exists(csrFilePath))
     {
         log<level::ERR>("CSR file doesn't exists",