Added possibility to compile without serial connection

Hi! I made a few changes you might want to consider pulling: 
1. I noticed that even though I just want to compile a file, blutil.py requires a serial connection to check the firmware version. I changed that, and updated the argument parser. 
2. Another change was to add an explicit check on the filename (and path). If the file doesn't exist, a sensible error is now given. It was not that clear what caused the error before.