Introduced by 0f3b89b.

If services.nixosManual.showManual is enabled and
documentation.nixos.enable is not, there is no
config.system.build.manual available, so evaluation fails. For example
this is the case for the installer tests.

There is however an assertion which should catch exactly this, but it
isn't thrown because the usage of config.system.build.manual is
evaluated earlier than the assertions.

So I split the assertion off into a separate mkIf to make sure it is
shown appropriately and also fixed the installation-device profile to
enable documentation.nixos.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @oxij

Changes the evaluation order in that it evaluates assertions before
warnings, so that eg. the following would work:

  { config, lib, ... }:

  {
    options.foo = lib.mkOption {
      type = lib.types.bool;
      default = true;
      description = "...";
    };

    options.bar = lib.mkOption {
      type = lib.types.bool;
      default = false;
      description = "...";
    };

    config = lib.mkMerge [
      (lib.mkIf config.bar {
        system.build.bar = "foobar";
      })
      (lib.mkIf config.foo {
        assertions = lib.singleton {
          assertion = config.bar;
          message = "Bar needs to be enabled";
        };
        systemd.services.foo = {
          description = "Foo";
          serviceConfig.ExecStart = config.system.build.bar;
        };
      })
    ];
  }

This is because the systemd module includes definitions for warnings
that would trigger evaluation of the config.system.build.bar definition.

The original pull request references a breakage due to the following:

  {
    services.nixosManual.enable = false;
    services.nixosManual.showManual = true;
  }

However, changing the eval order between asserts and warnings clearly is
a corner case here and it only happens because of the aforementioned
usage of warnings in the systemd module and needs more discussion.

Nevertheless, this is still useful because it lowers the evaluation time
whenever an assertion is hit, which is a hard failure anyway.