sysutils/grub2: how to avoid module loading when builtin?

[Frédéric Fauberteau <triaxx%NetBSD.org@localhost>]

Hi,

A look at dmesg show me:
NetBSD 8.0 (HYDRALISK) #1: Tue Jul 31 18:43:06 CEST 2018
	root@hydralisk:/usr/obj/sys/arch/amd64/compile/HYDRALISK
total memory = 3839 MB
avail memory = 3704 MB
kobj_checksyms, 1003: [ffs.kmod]: linker error: global symbol 
`ffs_vnodeop_opv_desc' redefined
kobj_checksyms, 1003: [ffs.kmod]: linker error: global symbol 
`ffs_snapshot_fini' redefined
*** idem ***
kobj_checksyms, 1003: [ffs.kmod]: linker error: global symbol 
`ufs_direnter' redefined
WARNING: module error: unable to load `ffs.kmod' pushed by boot loader, 
error 8

But ffs is builtin and the grub config generator unconditionally 
includes ffs.kmod.

Any idea on what could be the grep spell or the magic file that could be 
tested to know if a vfs is builtin/modularized?

Thx