Change 'readlink -f' to abspath method call to keep compatible with BSD ...

...readlink on OSX systems.

On the current master on OS X 10.6.8, I am able to patch my ROM but receive a syntax error when using a readlink call.

```
$ ./auto_patcher ~/Downloads/flinnycm101_vision_22.zip 

The Auto-Patcher v2.9.8

readlink: illegal option -- f
usage: readlink [-n] [file ...]
### verify tools ###
### verify ROM ###
### unzip patch tools ###
### unzip patch patches ###
```

Making the change to the relevant link in the script, using the compat method written for this purpose, gets rid of the error.