learning_gem5: Add code for hello-goodbye example

Adding more code from Learning gem5 Part II

See http://learning.gem5.org/book/part2/parameters.html

Change-Id: I9fe5655239e011c718c5cf5fd62bebcda66ea966
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/5021
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>