config: add --bootloader to fs.py and fs_bigLITTLE.py

This allows explicitly selecting which bootloader to use.

Before this commit, the bootloader had a fixed basename which
had to be present inside M5_PATH.

Change-Id: I02919207d6f175854017ae7b603d811da63d618e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23670
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

configs/common/Options.py

@@ -444,6 +444,8 @@ def addFSOptions(parser):
                 action="store_true", help="Enable stats dump at context "\
                 "switches and dump tasks file (required for Streamline)")
         parser.add_option("--vio-9p", action="store_true", help=vio_9p_help)
+        parser.add_option("--bootloader", action='append',
+                help="executable file that runs before the --kernel")
 
     # Benchmark options
     parser.add_option("--dual", action="store_true",