config, arm: Unify checkpoint path handling in bL configs

config, arm: Unify checkpoint path handling in bL configs

The vanilla bL configuration file and the dist-gem5 configuration file
use slightly different code paths when restoring from
checkpoints. Unify this by passing the parsed options to the
instantiate() method and adding an optional checkpoint keyword
argument for checkpoint directories (only used by the dist-gem5
script).

Change-Id: I9943ec10bd7a256465e29c8de571142ec3fbaa0e
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Sascha Bischoff <sascha.bischoff@arm.com>
Reviewed-by: Gabor Dozsa <gabor.dozsa@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2560
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Weiping Liao <weipingliao@google.com>