Set the firmware_class path in modprobe.d

Activation scripts are run before systemd is started, so unless users
are loading firmware in the initrd (which AFAICT we currently have no
support for) the previous /sys-based setting of firmware_class was
ineffective on boot.

Signed-off-by: Shea Levy <shea@shealevy.com>
gummiboot: Fix bug where old conf scripts for single-digit generations weren't removed

Signed-off-by: Shea Levy <shea@shealevy.com>