Set a stricter default set of USE-flags

This deviates from the default profile use flags mainly because
we don't need half of it.

It additionally allows us to build a minimal busybox image where
glibc can get decoupled from.
Split glibc from the busybox package

This change allows us to further separate busybox, shrinking its size.
The "downside" is that it now needs to be static. We loose roughly one
megabyte by doing so, but the upside is cleaner separation.

The glibc image is additionally smaller than its previous counterpart,
since the default USE-flag set is smaller (effectively removing pam).

Finally, change the dependency of s6 from busybox to glibc.
Merge pull request #16 from jbergstroem/decouple_glibc

Decouple glibc from busybox