additional regdb key directory

configname: CONFIG_CFG80211_EXTRA_REGDB_KEYDIR

Linux Kernel Configuration
└─>Networking support
└─>Wireless
└─>additional regdb key directory
In linux kernel since version 4.2 (release Date: 2015-08-30)  
If selected, point to a directory with DER-encoded X.509
certificates like in the kernel sources (net/wireless/certs/)
that shall be accepted for a signed regulatory database.

Note that you need to also select the correct CRYPTO_<hash> modules
for your certificates, and if cfg80211 is built-in they also must be.