Prevent building of SLES12 on openSUSE 13.1

Fixes #114
Fix message about unsupported building environment

Since the user is most likely interested where he can build his system
description the error messages refers to the build matrix section in our
man page.
Adapt man page to correct build matrix
SLES11 can't be build on SLES11 with machinery

Machinery doesn't run on SLES11 so there is no point listing this
as supported build host.
Make build matrix description more clear
Merge pull request #115 from SUSE/disable_sle12_building_on_131

Prevent building of SLES12 on openSUSE 13.1