Take a look at http://wiki.maemo.org/Devuan_on_N900. I had the same issue when using a kernel with the watchdogs in the modules, no initrd, and udev. What happens (in my case): no initrd, so when udev starts the loopback lo is not mounted; udev starts net.agent, which waits for lo and hangs the system for a while; that while is enough for the kernel to timeout (due to the absence of watchdogs - I admit I don't know this particular in detail) and call a system halt. Problem solved by building the watchdogs in the kernel; then you get a 30 seconds hang, but at last the system boots. If you also manage to get rid of udev (with all the cons this causes) you also get rid of the 30 seconds hang.