###### Motivation for this change
Update to latest upstream version
###### Things done
* [ ] Tested using sandboxing ([nix.useSandbox](http://nixos.org/nixos/manual/options.html#opt-nix.useSandbox) on NixOS, or option `sandbox` in [`nix.conf`](http://nixos.org/nix/manual/#sec-conf-file) on non-NixOS linux)
* Built on platform(s)
* [x] NixOS
* [ ] macOS
* [ ] other Linux distributions
* [ ] Tested via one or more NixOS test(s) if existing and applicable for the change (look inside [nixos/tests](https://github.com/NixOS/nixpkgs/blob/master/nixos/tests))
* [ ] Tested compilation of all pkgs that depend on this change using `nix-shell -p nix-review --run "nix-review wip"`
* [x] Tested execution of all binary files (usually in `./result/bin/`)
* [ ] Determined the impact on package closure size (by running `nix path-info -S` before and after)
* [ ] Ensured that relevant documentation is up to date
* [x] Fits [CONTRIBUTING.md](https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md).
###### Notify maintainers
cc @worldofpeace @tfc @jtraue
While renaming `networking.defaultMailServer` directly to
`services.ssmtp` is shorter and probably clearer, it causes eval errors
due to the second rename (directDelivery -> enable) when using e.g. `lib.mkForce`.
For instance,
``` nix
{ lib, ... }: {
networking.defaultMailServer = {
hostName = "localhost";
directDelivery = lib.mkForce true;
domain = "example.org";
};
}
```
would break with the following (rather confusing) error:
```
error: The option value `services.ssmtp.enable' in `/home/ma27/Projects/nixpkgs/nixos/modules/programs/ssmtp.nix' is not of type `boolean'.
(use '--show-trace' to show detailed location information)
```
NixOS 20.03 is built on kernel 5.4 and 19.09 is on 4.19, so we should update
this option to the highest value possible, per linked upstream instructions from
Amazon.
* nixos/nixpkgs.nix: Allow just using config in system
This assertion requires system to work properly. We might not have
this in cases where the user just sets config and wants Nixpkgs to
infer system from that. This adds a default for when this happens,
using doubleFromSystem.
* parens
- Avoid using overrides unless necessary
- Set platform and ARCH by default
- Don’t set dontConfigure unless absolutely necessary
- Use preBuild instead of overriding entire configurePhase
