Number of bytes to reserve for the extra certificate

configname: CONFIG_SYSTEM_EXTRA_CERTIFICATE_SIZE

Linux Kernel Configuration
└─>Cryptographic API
└─>Certificates for signature checking
└─>Number of bytes to reserve for the extra certificate
In linux kernel since version 4.1 (release Date: 2015-06-21)  
This is the number of bytes reserved in the kernel image for a
certificate to be inserted.