Add wrapper function for machine id

[brainelectronics]

Use function of https://github.com/brainelectronics/Micropython-ESP-WiFi-Manager/blob/b5d355cba0e479049a78a081bc99d5f3367d5e53/wifi_manager/wifi_manager.py#L69 to generate a unique id of specified length. Default is standard length. To be used in brainelectronics/MyEVSE-Webinterface#9

[brainelectronics]

Extend function to return also the last n character as follows

amount = abs(length) // uuid_len + (abs(length) % uuid_len > 0)
if length < 0:
    return (uuid * amount)[-length:]
else:
    return (uuid * amount)[:length]