west: runners/misc-flasher: fix passing extra args to script

The argparse module, by default, complains about non-defined
arguments. This prevents passing arguments prefixed with '-'
or '--' to the target script (e.g. calling another Python
script using argparse). This changes the misc-flasher script
so that any arguments not recognized by west will be passed
to the target script.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>