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/firmware_handler.cpp b/bmc/firmware_handler.cpp
index 9923d5b..917dbc4 100644
--- a/bmc/firmware_handler.cpp
+++ b/bmc/firmware_handler.cpp
@@ -33,8 +33,6 @@
 #include <string>
 #include <vector>
 
-using namespace phosphor::logging;
-
 namespace ipmi_flash
 {
 
@@ -46,7 +44,7 @@
     /* There must be at least one in addition to the hash blob handler. */
     if (firmwares.size() < 2)
     {
-        log<level::ERR>("Must provide at least two firmware handlers.");
+        std::fprintf(stderr, "Must provide at least two firmware handlers.");
         return nullptr;
     }
     if (transports.empty())