Additional X.509 keys for default system keyring

configname: CONFIG_SYSTEM_TRUSTED_KEYS

Linux Kernel Configuration
└─>Cryptographic API
└─>Certificates for signature checking
└─>Additional X.509 keys for default system keyring
In linux kernel since version 4.1 (release Date: 2015-06-21)  
If set, this option should be the filename of a PEM-formatted file
containing trusted X.509 certificates to be included in the default
system keyring. Any certificate used for module signing is implicitly
also trusted.

NOTE: If you previously provided keys for the system keyring in the
form of DER-encoded *.x509 files in the top-level build directory,
those are no longer used. You will need to set this option instead.