faft: Improve the readability of FAFTClient class

This CL does the following improvements:
 - Log the methods during dispatching instead per-method logging;
 - Reorder the methods by categories;
 - Rename system.get_gbb_flags to bios.get_gbb_flags.

BUG=chromium-os:36429
TEST=run firmware_FAFTSetup and passed.

Change-Id: I685aea5ffccbfb8eaef5237d5052e7b74f45ce9c
Reviewed-on: https://gerrit.chromium.org/gerrit/39158
Reviewed-by: Vic Yang <victoryang@chromium.org>
Commit-Ready: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>
diff --git a/server/cros/faftsequence.py b/server/cros/faftsequence.py
index 1519eeb..77b95e5 100644
--- a/server/cros/faftsequence.py
+++ b/server/cros/faftsequence.py
@@ -595,7 +595,7 @@
           clear_mask: A mask of flags to be cleared.
           set_mask: A mask of flags to be set.
         """
-        gbb_flags = self.faft_client.system.get_gbb_flags()
+        gbb_flags = self.faft_client.bios.get_gbb_flags()
         new_flags = gbb_flags & ctypes.c_uint32(~clear_mask).value | set_mask
 
         if (gbb_flags != new_flags):