s/::bt/::ipmi/g to be more correct

The code references bt everywhere for blocktransfer, to mean, inside the
IPMI packet itself.  However, those packets could be KCS and come in
through the same IPMI daemon.  Therefore, rename bt to ipmi for the data
handler type.

Change-Id: I5a460609fc09905b911b534ef6a1a13faa9cef29
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/firmware_handler.hpp b/firmware_handler.hpp
index cc7d2c0..fc66847 100644
--- a/firmware_handler.hpp
+++ b/firmware_handler.hpp
@@ -34,7 +34,7 @@
   public:
     enum FirmwareUpdateFlags : std::uint16_t
     {
-        bt = (1 << 8),   /* Expect to send contents over IPMI BlockTransfer. */
+        ipmi = (1 << 8), /* Expect to send contents over IPMI BlockTransfer. */
         p2a = (1 << 9),  /* Expect to send contents over P2A bridge. */
         lpc = (1 << 10), /* Expect to send contents over LPC bridge. */
     };