profile: ignore leading slashes in <description_file> field

Some versions of XBee Studio create .xpro files where the
`<description_file>` field has a leading slash
(such as `/XB3-24Z/XB3-24Z_1013-th.xml`).
Python's `zipfile` library does not normalize paths like
`radio_fw//XB3-24Z/XB3-24Z_1013-th.xml`, and so any such .xpro file
is treated as unparseable.

https://onedigi.atlassian.net/browse/XBPL-432

Author: mikewadsten

digi/xbee/profile.py

@@ -894,7 +894,7 @@ def _parse_xml_profile_file(self, zip_file):
             if fw_xml_file_element is None:
                 self._throw_read_exception(_ERROR_PROFILE_XML_INVALID
                                            % "missing firmware file element")
-            self._fw_xml_filename = fw_xml_file_element.text
+            self._fw_xml_filename = fw_xml_file_element.text.lstrip("/")
             # Store XML firmware file name.
             self._fw_xml_file = _FW_DIR_NAME + "/" + self._fw_xml_filename
             _log.debug(" - XML firmware file: %s", self._fw_xml_file)