It is recommended that users building their own kernels never configure as a module any driver that Debian doesn't configure as a module.
Anyway, essential modules for system operation should preferably be added to /etc/modules. They will be early-loaded by whatever gets to them first (be it the initramfs, or systemd, or sysvinit). Both initramfs-tools and dracut know to always add modules present in /etc/modules to the initramfs, and attempt to manually load them. Adding essential modules to /etc/initramfs-tools/modules works, but it is specific to initramfs-tools. -- Henrique Holschuh