add the USB library path as optional configuration parameter for the usb_transport

[danielhrisca]

The default USB library resolution in pyusb fails if the user needs to use a custom USB library, or if the USB library is not installed system wide. With this PR the user can give the absolute path of the USB library

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.