OK, mmc issue sorted out by adding
CONFIG_DMADEVICES=y
CONFIG_DMA_OMAP=y

This time Nemo does get started, but dies soon after (http://pastie.org/10433263). I guess watchdog shuts it off before DSME is able to start and prevent that.

Anyway - I now have a working serial console cable, and we have kernel that kinda boots. I hope to make some more progress in the following days (time permitting).