I just downloaded the latest CST tool 3.3.1 and found this in the documentation.
The /back_end-hsm directory contains the sources and headers necessary for replacing the open source cryptographic libraries with an implementation that can interact directly with a Hardware Security Module (HSM) by using the PKCS#11 interface definition. More detailed information can be found in the README file located in the directory.
Note: This add-on is deprecated and will be removed in the next CST release.
What will this be replaced with? Having private keys on the build machine is not a very safe practice.