src/platforms/common: factor out {amd,arm}64.nix

This commit creates a (currently empty) overlay list for things
common to each of the amd64 and arm64 platform-classes.

src/default.nix

@@ -31,11 +31,14 @@ let
       (self: { inherit lib nixpkgsOnBuildForBuild; })).overrideScope'
       (lib.foldl' lib.composeExtensions (_: prev: prev) ([base] ++ overlays));
 
+  common_amd64 = import ./platform/common/amd64.nix { inherit lib; };
+  common_arm64 = import ./platform/common/arm64.nix { inherit lib; };
+
 in {
   # one entry for each supported platform
-  kevin = ownerboot (import ./platform/kevin { inherit lib; });
-  am1i  = ownerboot (import ./platform/am1i { inherit lib; });
-  kgpe  = ownerboot (import ./platform/kgpe { inherit lib; });
+  kevin = ownerboot (import ./platform/kevin { inherit lib common_arm64; });
+  am1i  = ownerboot (import ./platform/am1i { inherit lib common_amd64; });
+  kgpe  = ownerboot (import ./platform/kgpe { inherit lib common_amd64; });
 
   em100 = nixpkgsOnBuildForBuild.callPackage ./util/em100 { };
 }

src/platform/am1i/default.nix

@@ -1,10 +1,11 @@
 { lib ? import <nixpkgs/lib>
 , hostPlatform ? lib.systems.examples.gnu64
+, common_amd64
 }:
 {
   inherit hostPlatform;
 
-  overlays = [(final: prev: {
+  overlays = common_amd64.overlays ++ [(final: prev: {
 
     platform_name = "am1i";
 

src/platform/common/amd64.nix

@@ -0,0 +1,7 @@
+{ lib ? import <nixpkgs/lib>
+}:
+
+{
+  overlays = [( final: prev: {
+  })];
+}

src/platform/common/arm64.nix

@@ -0,0 +1,7 @@
+{ lib ? import <nixpkgs/lib>
+}:
+
+{
+  overlays = [(final: prev: {
+  })];
+}

src/platform/kevin/default.nix

@@ -1,5 +1,6 @@
 { lib ? import <nixpkgs/lib>
 , hostPlatform ? lib.systems.examples.aarch64-multiplatform
+, common_arm64
 }:
 let
   # order matters!  this is also the order in which the modules are insmoded
@@ -36,7 +37,7 @@ let
 in {
   inherit hostPlatform;
 
-  overlays = [(final: prev: {
+  overlays = common_arm64.overlays ++ [(final: prev: {
     nixpkgsOnBuildForHost = prev.nixpkgsOnBuildForBuild.pkgsCross.aarch64-multiplatform;
 
     platform_name = "kevin";

src/platform/kgpe/default.nix

@@ -1,11 +1,12 @@
 { lib ? import <nixpkgs/lib>
 , hostPlatform ? lib.systems.examples.gnu64
+, common_amd64
 }:
 
 {
   inherit hostPlatform;
 
-  overlays = [(final: prev: {
+  overlays = common_amd64.overlays ++ [(final: prev: {
 
     platform_name = "kgpe";