tests: update linux boot tests

tests: update linux boot tests

This change updates the config scripts used by linux boot
tests to make them compatible with change that deprecated
LinuxX86System

JIRA: https://gem5.atlassian.net/browse/GEM5-440

Change-Id: I04beff2915c03a2c3a774351edbba60d7ff26867
Signed-off-by: Ayaz Akram <yazakram@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28427
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>