So you want to build just the cyanogenmod kernel, not the whole 50+GB world...

To build a system image, or just the kernel, e.g. the cyanogenmod wiki contains pretty exhaustive first-steps guide.

However, it doesn't say what to do to rebuild the kernel after modifying it and/or closing the session. This worked for me:

cd $cyanogen/android/system
. build/envsetup.sh #TODO: maybe this is done by breakfast already
breakfast mako 
mka bootimage
# done...
adb reboot bootloader
sudo `which fastboot` boot out/target/product/mako/boot.img