lsusb lists the fingerprint reader in the x240 as follows:
Bus 002 Device 003: ID 138a:0017 Validity Sensors, Inc.
There exists experimental driver support for this in a fork of libfprint for vfs5011 sensors, however you'll need to compile the driver yourself. To get the fingerprint sensor to work for lightdm login, su etc, follow these steps.
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Install fingerprint-gui:
sudo add-apt-repository ppa:fingerprint/fingerprint-gui