Then create one. The /boot directory on Raspbian – and I would presume any Pi oriented distro, although it is not necessarily the case – is a mount point for the first (vfat) partition. That partition is non-optional regardless of distro, and nothing in it has anything directly to do with anything outside of that partition. In fact, it does not even have to be mounted except to simplify updates via the package manager; it is never used once the kernel loads.
So “/boot/config.txt” really refers to a file in the boot partition; the other files there are the kernel, firmware, device tree overlays, etc. (although much of that, including config.txt, is optional).
The default config.txt on Raspbian is actually only one line and a lot of comments and commented out directives. The one line is for loading the audio driver, which evidently is not built in to the stock kernel.
Here’s what you need for the camera:
You can assign more GPU memory but not less. Reboot, then try raspistill. If it is not available in Ubuntu Core, it can be installed easily enough. Follow the directions here for adding /opt/vc:
raspistill and raspivid are in /opt/vc/bin. You’ll want to add that to your $PATH or else move them somewhere. You must tell the linker about the libraries using /etc/ld.so.conf(.d) and ldconfig.