Have you found the OpeWrt wiki pages?

https://wiki.openwrt.org/start?do=search&id=ipsec

I would be very curious about the performance of StrongSwan compared to OpenVPN. I am of the impression it should be much faster, especially on these resource constrained devices. It was beyond my comprehension when I looked at it back in time.

Are you building a client for use with a VPN service or a client\server setup?