envvar allows adding individual environment variables to the kernel's static environment without the overhead of pulling in a full file. envvar in a config looks like:
```
envvar "some_var=5"
```
All envvar-provided variables will be added after the `env` file is processed, so `envvar` keys that exist in the previous `env` will be overwritten by whatever value is set here in the kernel directly.
This has a caveat that quotes may not be included anywhere in the envvar string or a syntax error will be raised. This turns out to be par for the course, almost- when we strip lines in any `env` provided files, we remove any and all quotes anyways. The silent failure of `env` is turned into a more verbose error with `envvar`.