Allow users to specify their custom Linux kernel command. Change-Id: I1e88523c918369ea5dd86c088eca82471663e76a Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20900 Tested-by: kokoro <noreply+kokoro@google.com> Reviewed-by: Jason Lowe-Power <jason@lowepower.com> Maintainer: Jason Lowe-Power <jason@lowepower.com>