move implementation to ipmi file

Move the entrance point for the IPMI OEM handling to a separate file and
pass in the handler pointer via binding.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I4371ebee1c252ab6e9d8a7ff2389583919ecc996
diff --git a/psu.cpp b/psu.cpp
index a5abda5..3876fcf 100644
--- a/psu.cpp
+++ b/psu.cpp
@@ -18,7 +18,7 @@
 
 #include "errors.hpp"
 #include "handler.hpp"
-#include "main.hpp"
+#include "ipmi.hpp"
 
 #include <cstdint>
 #include <cstring>