pflash command line option parsing improvement

Today I accidentally typed

```
pflash -p PAYLOAD -p skiboot.lid.xz
```

when I obviously meant

```
pflash -P PAYLOAD -p skiboot.lid.xz
```

and pflash didn't give me an error about having two `-p` options.

After briefly looking at the code, it looks like the option parsing in general isn't fantastic and could use some work.