The ST-Link V2 clone update with U2F token firmware (https://github.com/gl-sergei/u2f-token - thanks to Sergei!). The 3d printed end cap to close modified case. U2F is acronym for Universal Second Factor - for strong authentication to Google, Facebook, etc. accounts.

In the end cap is a hole for micro-switch (D2F type - dimensions 12.6x5.7x6mm) - for "user presence". I used a D2F-01 switch from the broken computer mouse.

The second, bigger version of end cap - with separate button (black cap and orange button) is more ergonomic to use I suppose. No changes in electronics and micro switch. (STL files: st-link-u2f-token_v2.stl and st-link-u2f-token_v2_button.stl). Both versions FreeCAD files attached.

A port to upload U2F firmware showed on the photo (a second ST-Link clone is needed). An another photo shows connections PCB and the switch. A value of small resistor is between 1k and 3.3k.

Two ST-Link's are needed http://s.click.aliexpress.com/e/ckLMz9FF :

1. First ST-Link - for update / convert to U2F token.
2. Second ST-Link - to upload U2F firmware to the first one (or maybe more).

And:
D2F switch - dimensions 12.6x5.7x6mm
1k to 3.3k ohm small resistor
Isolation tape