npcx: Set hex_file automatically w/ npcx hex image

When using CONFIG_NPCX_IMAGE_OUTPUT_HEX, set the hex_file
runners_yaml_props_target property, so that west flash will flash the
correct file.

Change the cmake flash rules to populate the hex_file value in the
runners.yaml file if either CONFIG_BUILD_OUTPUT_HEX is enabled or some
cmake file set the runners_yaml_props_target hex_file property.

Update the npcx9m6f_evb instructions now that the filename is implicit.

Signed-off-by: Jeremy Bettis <jbettis@google.com>