move blob command enum to blob_interface header

Move the blob command enum to the blob_interface header that can be
safely included by other code.

Change-Id: If7c613cac9bdd460fc9a6cb3b59e4fc0dd3a861d
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/src/ipmiblob/blob_handler.cpp b/src/ipmiblob/blob_handler.cpp
index 2b79e19..039b7e0 100644
--- a/src/ipmiblob/blob_handler.cpp
+++ b/src/ipmiblob/blob_handler.cpp
@@ -48,7 +48,7 @@
 
     std::copy(ipmiPhosphorOen.begin(), ipmiPhosphorOen.end(),
               std::back_inserter(request));
-    request.push_back(command);
+    request.push_back(static_cast<std::uint8_t>(command));
 
     if (payload.size() > 0)
     {