I'm using vocore2, I need fast boot, so how to include systemd in initramfs in generated image?
systemd isn't available at all for OpenWrt.
ok, what is the other ways to improve fast loading or can you tell me how to have ramdisk image?
Initramfs images can be enabled at buildtime, I wouldn't expect those to boot much faster though (they are not meant to be used in production). Speed will be what it is, any improvements would come from case-by-case investigations and perf testing.
Discussion continues here How to have ramdisk image in openwrt