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

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>