The sys-kernel/dracut package was ported from the RedHat project and serves a similar tool for generating an initramfs. Since it is currently in ~arch for testing, users will need to accept it (through /etc/portage/package.accept_keywords) to emerge it. Before doing so, the variable
DRACUT_MODULES="lvm" should be added to /etc/portage/make.conf. Other modules may be desired, please refer to Dracut. Generally, the following command will generate a usable default initramfs.