This variable blocks the enable of online repositories and activates
the enable of RPMS.main repository in iso or rootfs.
The old name was misleading.

use/repo/main for vm targets will now create and enable the main
repository in /ALTLinux on rootfs.

We've got some parts of it in build-distro feature,
and some went to dev feature for no real reason.

But a bare installer might go without package base,
and LiveCDs other than live-builder might find local
repository useful given aufs2 root overlay.

Now the overall scheme is more straightforward:
- a distro:
  + asks that a package repo be included
  + cares to further add the packages to it
- a repo feature:
  + pulls in sub/main for it to happen
  + provides genbasedir script to create repo metadata
  + supplements live feature with repo configuration