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>

configs/example/arm/fs_bigLITTLE.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2016-2017, 2019-2020 ARM Limited
+# Copyright (c) 2016-2017, 2019-2021 ARM Limited
 # All rights reserved.
 #
 # The license below extends only to copyright in the software and shall
@@ -60,7 +60,6 @@ from devices import AtomicCluster, KvmCluster, FastmodelCluster
 
 
 default_disk = 'aarch64-ubuntu-trusty-headless.img'
-default_rcs = 'bootscript.rcS'
 
 default_mem_size= "2GB"
 
@@ -175,7 +174,7 @@ def addOptions(parser):
                         help="Hardware platform class")
     parser.add_argument("--disk", action="append", type=str, default=[],
                         help="Disks to instantiate")
-    parser.add_argument("--bootscript", type=str, default=default_rcs,
+    parser.add_argument("--bootscript", type=str, default="",
                         help="Linux bootscript")
     parser.add_argument("--cpu-type", type=str, choices=list(cpu_types.keys()),
                         default="timing",