log consistently via stderr

Most log messages in this codebase use stderr, while a few used
phosphor-logging.  Transition all to use stderr.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: Ic4cf619950275a2752f646f00974047092757010
diff --git a/bmc/main.cpp b/bmc/main.cpp
index 48af9d0..f9b1d7a 100644
--- a/bmc/main.cpp
+++ b/bmc/main.cpp
@@ -138,9 +138,7 @@
 
     if (!handler)
     {
-        using namespace phosphor::logging;
-
-        log<level::ERR>("Firmware Handler has invalid configuration");
+        std::fprintf(stderr, "Firmware Handler has an invalid configuration");
         return nullptr;
     }