Thanks to Debian GNU/kFreeBSD and Robert Millan

"commit b5b4042f4a5b585a16906dd4cc90b3403e356ad4
Author: rmh <rmh@FreeBSD.org>
Date:   Sat Feb 4 00:54:43 2012 +0000

    Add MK_SOURCELESS build option. Setting MK_SOURCELESS to "no" will disable
    kernel modules that include binary-only code.
    
    More fine-grained control is provided via MK_SOURCELESS_HOST (for native code
    that runs on host CPU) and MK_SOURCELESS_UCODE (for microcode).
    
    Reviewed by:        julian, delphij, freebsd-arch
    Approved by:        kib (mentor)
    MFC after:  2 weeks

Notes:
    svn path=/head/; revision=230972"

Caveat: src.conf entry does not affect the kernel itself.