Only show/build a package on the platforms listed in meta.platforms

The function ‘mkDerivation’ now checks whether the current platform
type is included in a package's meta.platform field.  If not, it
throws an exception:

  $ nix-build -A linux --argstr system x86_64-darwin
  error: user-thrown exception: the package ‘linux-3.10.15’ is not supported on ‘x86_64-darwin’

These packages also no longer show up in ‘nix-env -qa’ output.  This
means, for instance, that the number of packages shown on
x86_64-freebsd has dropped from 9268 to 4764.

Since meta.platforms was also used to prevent Hydra from building some
packages, there now is a new attribute meta.hydraPlatforms listing the
platforms on which Hydra should build the package (which defaults to
meta.platforms).

pkgs/top-level/release-lib.nix

@@ -119,14 +119,12 @@ rec {
 
   # May fail as much as it wishes, we will catch the error.
   processPackage = attrSet:
-    if attrSet ? recurseForDerivations && attrSet.recurseForDerivations then
+    if attrSet.recurseForDerivations or false then
       packagesWithMetaPlatform attrSet
-    else if attrSet ? recurseForRelease && attrSet.recurseForRelease then
+    else if attrSet.recurseForRelease or false then
       packagesWithMetaPlatform attrSet
     else
-      if attrSet ? meta && attrSet.meta ? platforms
-        then attrSet.meta.platforms
-        else [];
+      attrSet.meta.hydraPlatforms or (attrSet.meta.platforms or []);
 
 
   /* Common platform groups on which to test packages. */