configs: Enable DTB autogeneration in starter_fs.py

configs: Enable DTB autogeneration in starter_fs.py

This patch is removing hardcoded default DTBs in favour of common DTB
autogeneration.

Change-Id: I68fdc2a169bfa8e8657c9ed4e4e127957a08cca1
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15959
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>