configs: Do not require args.kernel to be set in baremetal.py

configs: Do not require args.kernel to be set in baremetal.py

This is allowing to us run baremetal.py with the --dtb-gen option
without needing to specify a --kernel argument

Change-Id: I98f1bc865d2f4e2230b1a85453efe83d95ec8a55
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28148
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>