configs: Remove default bootscript option for fs_bigLITTLE.py

configs: Remove default bootscript option for fs_bigLITTLE.py

Since the beginning fs_bigLITTLE has been pointing to a default

default_rcs = 'bootscript.rcS'

as a System.readfile parameter. That script is not present in
the gem5 repo and all the other fs scripts (starter_fs.py, fs.py
through Options.py) are using an emptry string as default
readfile param value.

We are hence aligning to the other scripts by removing this
default value

Change-Id: I20dc7714deae890d61706459c8d13bd8f5aac7a0
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38815
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>