[Tracking Requested - why for this release]: Fennec ESR68 transition

From https://docs.google.com/document/d/1oRPkwP3l7QzdQYj0Wn7d_3EfTZaakocA_i_pGKlG0dI/edit?ts=5cb4dd45#heading=h.m8n74cgxatdx:

Create mozconfigs for nightly builds on Beta after 68 merges there during the soft freeze week of May 6. These must keep the package name org.mozilla.fennec_aurora. These builds will also migrate to ESR68 eventually.
Nightly builds off Beta/ESR68 will not be able to use the NIGHTLY_BUILD ifdef since that is based on the version number. We may want to define a FENNEC_NIGHTLY ifdef for these builds to use for any work intended only for Nightly users.
Display version number to follow 68.Xa1 pattern.
Will need to run tests on these builds in addition to regular “release” builds.

Marking as blocking since this needs to happen before 68 goes to beta.

Hi Johan, to be clear, is this work you would like a build peer to take on or is it something that releng will be working on?

Hi Kim! Thanks for looking at this bug so quickly 🙂 I don't think I have enough knowledge about the build config implications. I know :RyanVM wanted to sync up with :nalexander about this topic. Forwarding the NI to Ryan.

I've filed bug 1547710 for adding the FENNEC_NIGHTLY define. I believe Johan was planning to work on the remaining items from the first comment.

Create mozconfigs for Fennec beta builds.
This also prevents mergeday scripts from modifying nightly mozconfigs on beta

https://hg.mozilla.org/mozilla-central/rev/486f55e81c57