Fix linux detection with buildroot

[bkuhls]

Inspired by
http://lists.busybox.net/pipermail/buildroot/2015-July/133312.html

[AchimTuran]

Hi @bkuhls
thanks for your contribution, but this is only the fork of asplib. Any new features should go into https://github.com/AchimTuran/asplib/ first and afterwards I create a new version here.

Would you create a new PR for that repo or should I pick your commit and push it to my repo?

[bkuhls]

You can pick the commit and push it to your repo if you like ;)

[AchimTuran]

@bkuhls
It's already picked ;-)
https://github.com/AchimTuran/asplib/blob/master/asplib_utils/os/asplib_base_os.h#L34

So I will close this PR.