Issue description

The logic in pkgs/stdenv/generic/check-meta.nix for checking the meta.platforms attribute currently compares meta.platforms against the stdenv.system attribute. In my cross-compilation environment stdenv.system is the double of my build platform, x86_64-linux.

It seems to me like this should rather be compared against stdenv.hostPlatform.system. Unfortunately, this attribute has slightly different semantics than stdenv.system. For instance, in the case of armv7l-hf-multiplatform it is arm-linux whereas the value expected by uboot is something like armv7l-linux.

How much detail should meta.system capture?

Pinging @Ericson2314.