bump(sys-kernel/bootengine): Updates for new dracut version

Dracut is now in portage-stable and DRACUT_MODULES is unused.
Update bootengine dependencies so we can use btrfs and mdadm in the
initrd. Drop kbd because I'm excluding out the terminfo module now.
Merge pull request #606 from marineam/dracut

bump(sys-kernel/bootengine): Updates for new dracut version