misc: Fix util/gem5img.py for new versions of sfdisk

misc: Fix util/gem5img.py for new versions of sfdisk

Newer versions of sfdisk have changed the format of the dump output,
as well as the options for partitioning a disk.

Updated the gem5img.py script to work with the new version of sfdisk.
The script should still work with older versions of sfdisk, but this
has not been tested (see https://askubuntu.com/a/819614).

Tested on Ubuntu 18.04.2 LTS with sfdisk from util-linux 2.31.1.

Change-Id: I1197ecacabdd7caaab00327977fb9ab6eae06654
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29472
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>