Makefile: add phony targets for generating all assets of a training

[tleb]

Sometimes you want to build-test all training content once you modified some slides. Add PHONY targets:

⟩ # generate slides/labs/agendas/tarballs for all three variants 
⟩ make linux-kernel
⟩
⟩ # same thing for the audio training:
⟩ #   full-audio-slides.pdf audio-agenda.pdf audio-fr-agenda.pdf
⟩ #   audio-online-agenda.pdf audio-online-fr-agenda.pdf
⟩ make audio

[tleb]

Just to clarify: I created this pull request to ask for feedback rather than push directly. I am not certain everyone agrees with the change (even though I don't see many cons to it).

[Tropicao]

Hi @tleb , I agree with the change, and I like it, from a trainer POV

[tleb]

Thanks for the review @Tropicao! I would like parallel compilation of agendas to be working before we merge this. Mostly a selfish desire because my system has MAKEFLAGS=-j20 globally. :-) Agendas look like the only place where our parallel compilation is broken, see #268 (comment)

[Tropicao]

Hi @tleb,
I am still pretty interested in getting your PR merged. You mentioned that you would like parallel build fix merged first, but it looks stalled. I also fail to see a hard requirement between the two PRs. If they are not indeed tightly coupled, can we move forward with this one ?

[tleb]

My comment above explains why I was interested in fixing parallel builds first: I do parallel builds by default because I have MAKEFLAGS=-j19 set globally. Anyway we can merge the patch to avoid keeping it stalled