esptool_py: use passed offset and image when template is given

esptool_py: use passed offset and image when template is given

esptool_py defines command `esptool_py_flash_project_args` that
generates arg file for esptool.py. Two of the arguments are the offset
and image, which are not being used when a template file is given.
This commit makes variables OFFSET and IMAGE available to the template
file, which will holds the value of the offset and image arguments to
`esptool_py_flash_project_args`.