Use a util helper to do prefix/suffix removal.

cloudinit/handlers/boot_hook.py

@@ -48,11 +48,9 @@ class BootHookPartHandler(handlers.Handler):
     def _write_part(self, payload, filename):
         filename = util.clean_filename(filename)
         filepath = os.path.join(self.boothook_dir, filename)
-        contents = util.dos2unix(payload)
-        if contents.startswith(BOOTHOOK_PREFIX):
-            real_start = len(BOOTHOOK_PREFIX) + 1
-            contents = contents[real_start:]
-        util.write_file(filepath, contents, 0700)
+        contents = util.strip_prefix_suffix(util.dos2unix(payload),
+                                            prefix=BOOTHOOK_PREFIX)
+        util.write_file(filepath, contents.lstrip(), 0700)
         return filepath
 
     def handle_part(self, _data, ctype, filename,  # pylint: disable=W0221

cloudinit/util.py

@@ -1530,6 +1530,14 @@ def shellify(cmdlist, add_header=True):
     return content
 
 
+def strip_prefix_suffix(line, prefix=None, suffix=None):
+    if prefix and line.startswith(prefix):
+        line = line[len(prefix):]
+    if suffix and line.endswith(suffix):
+        line = line[:-len(suffix)]
+    return line
+
+
 def is_container():
     """
     Checks to see if this code running in a container of some sort