pyocd flashing does not work

### Description
- Type: Bug
- Related Issue: #149

---------------------------------------------------------------
## Bug

**mbed-flasher version**
v0.9.2

probably same happens with v0.10.0

**Expected Behavior**
flashing using `--method pyocd` flash the device

**Actual Behavior**
Flashing fails with message: `[ERROR](mbed-flasher): Failed: PyOCD is missing`

**Steps to Reproduce**
```
$ mbedflash flash -i app.bin --tid 0240000032254e450058800bb52900293f31000097969900 pyocd
[ERROR](mbed-flasher): Failed: PyOCD is missing
```

Start with the CLI path for `mbedflash flash` and the `--method pyocd` handling, then reproduce the command from the issue to inspect why it reports that PyOCD is missing. Done means the command can flash the specified device successfully without that error.

Written by the indexing model from the issue text.