igh-ethercat: use release tarball instead of mercurial branch

The Mercurial branch may change, which leads to non-reproducible builds.
So download a released version instead.

Also add configure option for e1000e.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>