sanitycheck: Make --hardware-map and --west-flash together

When passing a using --hardware-map, sanitycheck would ignore extra
args passed via --west-flash, such as --west-flash="--erase".

This is because the command with the extra args was overwritten by the
command with the runner info from the hardware map.

This patch merges those code paths so they are aware of each other and
of --west-runner.

Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>