src/platform/*: move hostPlatform into the packageset

This commit makes `hostPlatform` part of the packageset for more
consistent handling.  A top-level `hostPlatform` argument is exposed
to the caller, in case they want to customize the `hostPlatform`
(compiler flags, etc).

The `hostPlatform` argument is inherited into the ownerboot package
set, where it will be overridden by `src/platform/*.nix` if it has
not been set explicitly.

Making `hostPlatform` part of the package set allows for more
sophisticated overriding schemes, for example adding additional
compiler flags or sanity-checking the flags that the user has
provided.

src/default.nix

@@ -1,11 +1,12 @@
 { lib     ? import <nixpkgs/lib>
 , pkgsFun ? import <nixpkgs>
+, hostPlatform ? null         # if left `null` this will be set based on the ./platform/
 }:
 let
   nixpkgsArgs = { config.allowNonSource = false; };
   nixpkgsOnBuildForBuild  = pkgsFun nixpkgsArgs;
 
-  ownerboot = { hostPlatform?null, overlays }: let
+  ownerboot = { overlays }: let
     base = final: prev: {
       inherit hostPlatform;
       nixpkgsOnBuildForHost = pkgsFun

src/platform/am1i/default.nix

@@ -1,10 +1,7 @@
 { lib ? import <nixpkgs/lib>
-, hostPlatform ? lib.systems.examples.gnu64
 , common_amd64
 }:
 {
-  inherit hostPlatform;
-
   overlays = common_amd64.overlays ++ [(final: prev: {
 
     platform_name = "am1i";

src/platform/common/amd64.nix

@@ -3,5 +3,9 @@
 
 {
   overlays = [( final: prev: {
+    hostPlatform =
+      if prev.hostPlatform != null
+      then prev.hostPlatform
+      else lib.systems.examples.gnu64;
   })];
 }

src/platform/common/arm64.nix

@@ -3,5 +3,9 @@
 
 {
   overlays = [(final: prev: {
+    hostPlatform =
+      if prev.hostPlatform != null
+      then prev.hostPlatform
+      else lib.systems.examples.aarch64-multiplatform;
   })];
 }

src/platform/kevin/default.nix

@@ -1,5 +1,4 @@
 { lib ? import <nixpkgs/lib>
-, hostPlatform ? lib.systems.examples.aarch64-multiplatform
 , common_arm64
 }:
 let
@@ -35,7 +34,6 @@ let
     "drivers/net/wireless/marvell/mwifiex/mwifiex.ko"
   ];
 in {
-  inherit hostPlatform;
 
   overlays = common_arm64.overlays ++ [(final: prev: {
     nixpkgsOnBuildForHost = prev.nixpkgsOnBuildForBuild.pkgsCross.aarch64-multiplatform;

src/platform/kgpe/default.nix

@@ -1,11 +1,8 @@
 { lib ? import <nixpkgs/lib>
-, hostPlatform ? lib.systems.examples.gnu64
 , common_amd64
 }:
 
 {
-  inherit hostPlatform;
-
   overlays = common_amd64.overlays ++ [(final: prev: {
 
     platform_name = "kgpe";