Custom boot screen feature improvement

[jbrazio]

This PR improves the custom boot screen handling by adding the configuration as an optional section on Configuration_adv.h (Please pay attention that not all Configuration_adv.h files were updated, look at the vendor notice section near the SHOW_BOOTSCREEN directive on every Configuration.h).

We no longer require a centralized custom bitmap file, each example configuration can carry inside their own custom boot screen.

Marlin boot screen has now a customizable timeout.

By default the second LCD line now shows the marlinfw.org website address.

[thinkyhead]

I'm thinking it would work better (for our many configurations) if this is just an optional #include. Keep it simple, like this:

//#define CUSTOM_BOOTSCREEN
#if ENABLED(CUSTOM_BOOTSCREEN) && ENABLED(SHOW_BOOTSCREEN)
  #include "custom_bootscreen.h"
#endif

…and this data can live in custom_bootscreen.h alongside the example config, uncommenting CUSTOM_BOOTSCREEN for those configs that include one, so the compiler will throw an error if you forget to move over the bootscreen file.

[jbrazio]

Could go both ways, but the idea was really to simplify.
This means IMO not to add additional files, thus reusing exactly the same concept we have for Deltas. Their config files are the only ones having some sections which are dedicated to them.

As this feature targets vendors we won't be seeing much of it on the wild I guess.

[thinkyhead]

And yet… I don't think we should have such a blob (with an include) in the middle of Configuration_adv.h. So even if it is only this one file, I strongly favor the extra file and the #include to minimize the amount of cruft in the config file. And it does make the existence of the custom boot screen more "conspicuous" in the folder, which might catch attention of other vendors thinking of the same idea.

Heh-heh. We should make a few ourselves, starting with Printrbot.