configs: Remove default kernel value from system creation

configs: Remove default kernel value from system creation

Kernel was being set using a placeholder and then assigned the
correct value. This would generate the following error if the
placeholder file did not exist:
'IOError: Can't find file <placeholder> on path'

This patch follows the same directions of commit
12eca7ac04ae1ba559bf322b5c625513929d369d and removes the default
values, forcing the user to properly configure the kernel.

Change-Id: I0eb45d12eda6b6efe9a3fe118996b640844a7b34
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/11850
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>