KVS_PATH also use env var

Add suggestions by @locnnil

Author: jpm-canonical

src/platform/Linux/CHIPPlatformConfig.h

@@ -66,6 +66,4 @@ using CHIP_CONFIG_PERSISTED_STORAGE_KEY_TYPE = const char *;
 
 // ==================== Security Configuration Overrides ====================
 
-#ifndef CHIP_CONFIG_KVS_PATH
-#define CHIP_CONFIG_KVS_PATH "/tmp/chip_kvs"
-#endif // CHIP_CONFIG_KVS_PATH
+#define CHIP_DEFAULT_CONFIG_KVS_FILE_NAME "chip_kvs"

src/platform/Linux/PosixConfig.cpp

@@ -97,7 +97,8 @@ ChipLinuxStorage * PosixConfig::GetStorageForNamespace(Key key)
 
 CHIP_ERROR PosixConfig::Init()
 {
-    return PersistedStorage::KeyValueStoreMgrImpl().Init(CHIP_CONFIG_KVS_PATH);
+    std::string filePath = GetFilePath(CHIP_DEFAULT_CONFIG_KVS_FILE_NAME);
+    return PersistedStorage::KeyValueStoreMgrImpl().Init(filePath.c_str());
 }
 
 CHIP_ERROR PosixConfig::ReadConfigValue(Key key, bool & val)
@@ -453,17 +454,17 @@ bool PosixConfig::ConfigValueExists(Key key)
     return storage->HasValue(key.Name);
 }
 
-std::string PosixConfig::GetFilePath(std::string defaultFileName)
+std::string PosixConfig::GetFilePath(const std::string &defaultFileName)
 {
     // Match what GetFilename in ExamplePersistentStorage.cpp does.
-    const char * dir = getenv("TMPDIR");
+    const char *dir = std::getenv("TMPDIR");
     if (dir == nullptr)
     {
         dir = "/tmp";
     }
-    std::string storageDir = dir;
-
-    return storageDir + "/" + defaultFileName;
+    std::filesystem::path storageDir(dir);
+    std::filesystem::path filePath = storageDir / defaultFileName;
+    return filePath.string();
 }
 
 CHIP_ERROR PosixConfig::EnsureNamespace(const char * ns)

src/platform/Linux/PosixConfig.h

@@ -25,6 +25,7 @@
 
 #include <functional>
 #include <string>
+#include <filesystem>
 #include <inttypes.h>
 
 #include <lib/core/CHIPError.h>
@@ -112,7 +113,7 @@ class PosixConfig
 
 private:
     static ChipLinuxStorage * GetStorageForNamespace(Key key);
-    static std::string GetFilePath(std::string defaultFileName);
+    static std::string GetFilePath(const std::string &defaultFileName);
 };
 
 struct PosixConfig::Key