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/os-specific/linux/kernel/generic.nix

@@ -40,9 +40,6 @@
 , ...
 }:
 
-assert stdenv.system == "i686-linux" || stdenv.system == "x86_64-linux"
-  || stdenv.isArm || stdenv.system == "mips64el-linux";
-
 assert stdenv.platform.name == "sheevaplug" -> stdenv.platform.uboot != null;
 
 let