read_fru_data: move and rename constants

Moved the DBus constants, used for reading FRU records
from the Inventory, to the read_fru_data.hpp header file.
Renamed these constants to comply with the
lowerCamelCase convention.
Moved and renamed constants remain within the ipmi::fru namespace.

Change-Id: If32c4a9c176453c414d9813c801ecc11ff81c038
Signed-off-by: Kirill Pakhomov <k.pakhomov@yadro.com>
diff --git a/read_fru_data.hpp b/read_fru_data.hpp
index d3268b8..d89646c 100644
--- a/read_fru_data.hpp
+++ b/read_fru_data.hpp
@@ -10,6 +10,12 @@
 {
 using FRUId = uint8_t;
 using FRUAreaMap = std::map<FRUId, FruAreaData>;
+
+static constexpr auto xyzPrefix = "/xyz/openbmc_project/";
+static constexpr auto invMgrInterface = "xyz.openbmc_project.Inventory.Manager";
+static constexpr auto invObjPath = "/xyz/openbmc_project/inventory";
+static constexpr auto propInterface = "org.freedesktop.DBus.Properties";
+
 /**
  * @brief Get fru area data as per IPMI specification
  *