Homogenize configuration option name.

Rename '--enable-aom-highbitdepth' to '--enable-highbitdepth'

Change-Id: I1de13c3508c30c552532993419d8ace326142ab6
diff --git a/tools_common.c b/tools_common.c
index 1169594..3530210 100644
--- a/tools_common.c
+++ b/tools_common.c
@@ -260,7 +260,7 @@
 }
 
 // TODO(debargha): Consolidate the functions below into a separate file.
-#if CONFIG_AOM_HIGHBITDEPTH
+#if CONFIG_HIGHBITDEPTH
 static void highbd_img_upshift(aom_image_t *dst, aom_image_t *src,
                                int input_shift) {
   // Note the offset is 1 less than half.
@@ -451,4 +451,4 @@
     lowbd_img_downshift(dst, src, down_shift);
   }
 }
-#endif  // CONFIG_AOM_HIGHBITDEPTH
+#endif  // CONFIG_HIGHBITDEPTH