releasetools: Add common.ZipDelete().

We have been shelling out to 'zip -d' to delete existing ZIP entries in
add_img_to_target_files.py. This CL moves the function into common.py,
and calls that for the similar work in ota_from_target_files.py. This CL
also adds unittests for the newly added function.

Test: `m dist`
Test: python -m unittest test_common
Test: ota_from_target_files.py generates identical packages w/ and w/o
      the CL (so we know the streaming property computation is intact).
Test: Run 'add_img_to_target_files.py -a' that triggers a call to
      ReplaceUpdatedFiles().
Change-Id: Icaa6c3ea3ee2166023f78fa79275295f837ea842

tools/releasetools/common.py

@@ -1147,6 +1147,28 @@ def ZipWriteStr(zip_file, zinfo_or_arcname, data, perms=None,
   zipfile.ZIP64_LIMIT = saved_zip64_limit
 
 
+def ZipDelete(zip_filename, entries):
+  """Deletes entries from a ZIP file.
+
+  Since deleting entries from a ZIP file is not supported, it shells out to
+  'zip -d'.
+
+  Args:
+    zip_filename: The name of the ZIP file.
+    entries: The name of the entry, or the list of names to be deleted.
+
+  Raises:
+    AssertionError: In case of non-zero return from 'zip'.
+  """
+  if isinstance(entries, basestring):
+    entries = [entries]
+  cmd = ["zip", "-d", zip_filename] + entries
+  proc = Run(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
+  stdoutdata, _ = proc.communicate()
+  assert proc.returncode == 0, "Failed to delete %s:\n%s" % (entries,
+                                                             stdoutdata)
+
+
 def ZipClose(zip_file):
   # http://b/18015246
   # zipfile also refers to ZIP64_LIMIT during close() when it writes out the