configure: Enable out-of-tree iotests

In order to allow out-of-tree iotests, create a symlink for the check
script in the build tree.

While doing so, also write configured options relevant to the iotests to
common.env in the build tree; currently, this is the command to invoke
Python 2.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
