periph-smoketest: various usability improvements

This makes the tool more pleasant to use for newcomers and this makes flag
parsing more coherent.

- Print help on error.
- Consistently use NewFlagSet() everywhere with Usage() that prints nicer help
  page.
- Move code around for the ones that were not layed out in the same way as the
  rest.

This is a breaking change for gpiosmoketest, since it is switched from
undocumented positional arguments to flags.

devices/bmxx80/bmx280smoketest/bmx280smoketest.go

@@ -37,14 +37,14 @@ func (s *SmokeTest) Description() string {
 }
 
 // Run implements the SmokeTest interface.
-func (s *SmokeTest) Run(args []string) (err error) {
+func (s *SmokeTest) Run(f *flag.FlagSet, args []string) (err error) {
-	f := flag.NewFlagSet(s.Name(), flag.ExitOnError)
 	i2cID := f.String("i2c", "", "I²C bus to use")
 	i2cAddr := f.Uint("ia", 0x76, "I²C bus address to use; either 0x76 (BMx280, the default) or 0x77 (BMP180)")
 	spiID := f.String("spi", "", "SPI port to use")
 	record := f.Bool("r", false, "record operation (for playback unit testing)")
 	f.Parse(args)
 	if f.NArg() != 0 {
+		f.Usage()
 		return errors.New("unrecognized arguments")
 	}
 

devices/ssd1306/ssd1306smoketest/ssd1306smoketest.go

@@ -51,10 +51,8 @@ func (s *SmokeTest) Description() string {
 }
 
 // Run implements the SmokeTest interface.
-func (s *SmokeTest) Run(args []string) (err error) {
+func (s *SmokeTest) Run(f *flag.FlagSet, args []string) (err error) {
 	s.delay = 2 * time.Second
-
-	f := flag.NewFlagSet(s.Name(), flag.ExitOnError)
 	i2cID := f.String("i2c", "", "I²C bus to use")
 	spiID := f.String("spi", "", "SPI port to use")
 	dcName := f.String("dc", "", "DC pin to use in 4-wire SPI mode")
@@ -66,6 +64,7 @@ func (s *SmokeTest) Run(args []string) (err error) {
 	record := f.Bool("record", false, "record operation (for playback unit testing)")
 	f.Parse(args)
 	if f.NArg() != 0 {
+		f.Usage()
 		return errors.New("unrecognized arguments")
 	}