Add goenv (and helper) target(s). (#2740)
Add goenv (and helper) target(s). (#2740)

The new `goenv` make target will generate the go/vendor links, as well as
in-tree links to generated files.

Also:
- Fix up the phony target list
- Remove go/vendor in the `clean` target.
Oncilla/scion,Oncilla/scion,netsec-ethz/scion,Oncilla/scion,netsec-ethz/scion,netsec-ethz/scion,netsec-ethz/scion,Oncilla/scion