build-rootfs: Pre-generate tmpfiles

Some *tmpfiles.d(5)* entries specify paths in the immutable root
filesystem.  These need to be created at build time to prevent
*systemd-tmpfiles-setup.service* from failing at runtime.

build-rootfs.sh

@@ -67,6 +67,8 @@ fi
 
 rm -f /mnt/gentoo/lib/tmpfiles.d/provision.conf
 
+systemd-tmpfiles --root=/mnt/gentoo -E --exclude-prefix=/var --create
+
 systemctl preset-all --root=/mnt/gentoo
 rm -f /mnt/gentoo/lib/systemd/system/sysinit.target.wants/ldconfig.service