basically after that you need a feeds.conf that points to the ceropackages-3.10 repo, and you need to repeatedly
copy env/config-wndr37* .config
and keep doing a ./scripts feeds install `cat env/packages.list env/override.list`
until your .config matches the one in .config. You may need to manually add or subtract a package via the feeds install utility.
It's really a PITA to do the first time. It does sound saner for me to try adding the two desired lines to the makefile to recognise the 3800ch and see what happens.