add(disk_util): Rename cgpt.py to disk_util.

Use a more generic name so cgpt.py and disk_layout_util.sh can be merged
into one tool. Wrappers around wrappers around wrappers are tedious.

build_library/disk_layout_util.sh

@@ -2,7 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-CGPT_PY="${BUILD_LIBRARY_DIR}/cgpt.py"
+CGPT_PY="${BUILD_LIBRARY_DIR}/disk_util"
 
 cgpt_py() {
   if [[ -n "${FLAGS_adjust_part-}" ]]; then