configs: root, platform options in fs bigLITTLE

configs: root, platform options in fs bigLITTLE

(1) Two new options are added to fs_bigLITTLE.py:
    - "root": disk/partition containing the rootfs (def. "/dev/vda1")
    - "machine-type": hardware platform class (def. "VExpress_GEM5_V1")
        + Accepts platform classes from PlatformConfig
(2) Default kernel is not available in public uploads, force the user
    to provide its own kernel instead of crashing.

Change-Id: I88283ae12cd7289e15b9277ea2cc382e9136f11c
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20148
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>