[fpv] Support --flag fusesoc option in FPV script
[fpv] Support --flag fusesoc option in FPV script

This is already supported in dvsim, but in case users are still using
the `fpv` batch script, this PR adds a switch to support --flag option
to decide if we want to run top-level file sets or IP level.

Signed-off-by: Cindy Chen <399ad771a111eac8923348b1e2af093034e5d4f1@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan