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>