I've built 3.10.6 (I was running 3.10.5, and wanted something close, but with no danger of a name-space clash).
All I've done is changed this in kirkwood-3.10.6.config
#GML - CONFIG_SERIAL_OF_PLATFORM is not setand I now have a working serial console again!
Well, I got a full set of boot up messages. I didn't get the console prompt until I re-enabled the TO/getty line in inittab and ran "telinit q". (I'd disabled as it was producing message about re-=spawning too quickly.). That all works now.
It still doesn't auto-boot on power-on, though :-(.
PS: I forgot to mention that the nice thing about using VirtualBox+qemu is that you can end up with a large memory, multi-CPU Sheevaplug (so make -j2, etc.), and can pause the while thing in mid kernel-build, switch everything off and then start again hours later, picking up in mid-compilation.