fix(coreos-base/gmerge): Don't create /etc/make.profile

This conflicts with /etc/portage/make.profile in the chroot, generating
lots of warnings. This was always a weird hack anyway. Instead lets
start configuring portage properly in dev images, this will be handled
in the build scripts.
Merge pull request #318 from marineam/gmerge

fix(coreos-base/gmerge): Don't create /etc/make.profile