config: Add a --checkpoint-dir argument to read_config.py.
config: Add a --checkpoint-dir argument to read_config.py.

This argument lets the user restore a checkpoint after loading
simulator state from config.ini.

Change-Id: I6e0630d75b798a1d2536e2408660843f57f46c4b
Reviewed-on: https://gem5-review.googlesource.com/6941
Reviewed-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Maintainer: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>

TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5