Looking at a similar issue on a custom system running systemd it seems this may be because "CONFIG_DEVTMPFS" is not selected in the kernel configuration (notice this is the first thing on sciurus's list of kernel requirements for running systemd).
To now figure out how to correct this, start make menuconfig and use the search feature (press /) to look for "DEVTMPFS". The first hit is:
Symbol: DEVTMPFS [=y] Type : boolean Prompt: Maintain a devtmpfs filesystem to mount at /dev Location: -> Device Drivers (1) -> Generic Driver Options Low and behold, there is such an option in Device Drivers->Generic Driver Options.
As vonbrand says, you are best off starting with a configuration that matches what you have now. This can probably be found gzipped as /proc/config.gz. Copy that out and gunzip -c config.gz > .config and use that file as the preliminary .config in your source tree (more detail in step #2 here).