Make updates: coverage stats and `notice-up-to-date` part of the verify build stage

This MR does the following:

• Adds a make cover target to add coverage
• Symlinks the GOPATH rather than copying, inspired by https://github.com/cloudflare/hellogopher. This leads to less problems with stale GOPATHs.
• make verify now includes notice-up-to-date so that I don't forget
• Does not need to delete NOTICE in order to run notice-up-to-date
• Ensures that make notice-update-to-date works even when you have uncommitted changes in your working copy (same as other verify changes)
• Go licences are not statically coded