Support the new firmware info mechanism

[marcelstoer]

Build support for nodemcu/nodemcu-firmware#2830 into build-esp8266.

Set the environment variable USER_PROLOG.

@HHHartmann do you think it's feasible/acceptable to just remove 70% of the stuff in build-esp8266 and thereby no longer altering the welcome message for the frozen 1.5.x branch? It would be a relief to get rid of all that code 😉

If I only commit this change after the firmware's upcoming master drop it wouldn't affect developers on the master branch.

[HHHartmann]

I would give it some time before removing the 70% thingy. Not everyone updates to the newest master all the time. Maybe one or two releases.

[marcelstoer]

Hhhmm, alright...then indeed I'll need to create this one big if-else probing for NODE_VERSION_LONG or tools/update_buildinfo.sh as discussed before.

[HHHartmann]

You could also rely on the# define trick for the time being. Setting USER_PROLOG won't hurt anybody.

[HHHartmann]

But then you would need another docker image update