configs: Make --disk-image optional in baremetal.py

configs: Make --disk-image optional in baremetal.py

Since the script could be used to run baremetal applications, we don't
have to enforce the presence of a disk image

Change-Id: I511515361cfd7a2e06ede0df3ddcc595de15f38b
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26992
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>