configs: fix forwarding of --bootloader to fs.py

configs: fix forwarding of --bootloader to fs.py

Since I02919207d6f175854017ae7b603d811da63d618e the fs.py --bootloader
option can be used to select a bootloader explicitly.

Ia3d863db276a023b6a2c7ee7a656d8142ff75589 forgot to pass that parameter
likely due to a rebase issue and it gets searched in M5_PATH.

Change-Id: Ic4cf3ccf041e1c34eac7753424fe842bd34a77f9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26724
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>