stdlib: Create 'KernelDiskWorkload' abstract class

The purpose of this class is to:

* Create a standard way to set a typical kernel/disk image workload.
* Cleans up the manner in which readfile values/contents are set.
* Allows a user to specify their own kernel arguments, but still sets
sensible defaults.

As of this commit, this interface has been added to the X86Board and the
RISCVBoard.

Change-Id: I34f4c2b829f1ae5c1cae12039436cbb345a89d09
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/51949
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
11 files changed