change positional (simple/pyocd/edbg) method to be as optional method

## Flashing a device using pyOCD

```
C:\>mbedflash flash C:\path_to_file\myfile.bin --tid 0240000033514e45003f500585d4000ae981000097969900 -t K64F simple
```

Change positional method to optional method so that user could give a --method command to specify preferred method.

```
--method pyocd
```

Start at the command-line argument parsing for the flash command and inspect how the current positional simple, pyocd, or edbg method is handled. Make the method selectable with --method, then verify that the example command works with --method pyocd and that the help text reflects the new usage.

Written by the indexing model from the issue text.