Fixed OpenWRT images for WD N600 & N750

I noticed recently that images I’d generated for the WD N600 / N750 weren’t booting properly. It seems from r39891 built on the 13th March that something changed causing the image to never actually work on the router after being flashed.

I spent a bit of time yesterday debugging this and now as of r40004 things are back in order and working correctly.

Sorry if this caught anyone out!