newt: Allow third linker script to maximize split image size

[hasty]

This resolves a limitation of the split image build process, allowing for larger builds.

Prior to this change, the split build process worked like so:

1. Build the app image, linking against the regular "bsp.linkerscript" ld script.
2. Build the loader image, also linking against the regular "bsp.linkerscript" ld script.
3. Compare the resulting elfs from app and loader, look for common functions, and substitute their addresses from the loader's elf into the app's elf.
4. Re-link the app against the special "bsp.part2linkerscript" ld script, which maps everything into slot 2's address space, except for those addresses already mapped into the loader's elf in step 3 (and also do some hardware-specific monkeying, if needed).

That doesn't use the split image setup to its full potential, though. The regular linker script needs to enforce that the resulting image fits inside the provided flash area, but in the case of the build in step 1, this flash area is less than what is actually available, as it counts the various symbols that will be re-linked and eliminated from the image in step 4. That artificially limits the size of the app to its own code, plus the shared code, minus any non-overlapping code in the loader image. In reality, it has (almost) the entirely of both flash areas to play with, as long as the portion shared with the loader doesn't overflow in step 2, and it doesn't overstep the various needs of the boot loader (though existing scripts don't account for that as it stands).

This PR changes step 1 to use an optional third linker script specified by "bsp.wholeapplinkerscript". This allows a bsp to specify an initial app linker script that allows the full use of both image slots; in the absence of such a script, it falls back on existing behavior. The resulting app image may not be workable if too much of it consists of dependencies on loader code, but that will be revealed later by a failure in step 3.

It also changes the name of the setting in step 4 to "bsp.splitapplinkerscript", which is more accurate and implies the connection between the two settings ("bsp.part2linkerscript" was misleading anyway, as technically that was stage 3 of the linking process). It maintains backwards compatibility with existing bsp.yml files that still use "bsp.part2linkerscript", but in the presence of both, prefers "bsp.splitapplinkerscript".

[sjanc]

@ccollins476ad this looks like useful feature, any chance you could have a look at this?

[utzig]

GH workflows won't run out of the box, need to first enable it with Apache; I think it'd be better to drop those CI changes and just leave it run on Travis like it is doing already.

[hasty]

I apologize; I submitted this pull request a year ago, and not much came of it, so I forgot that I had done so. Months later, I checked in a GitHub action to build the project on my master branch; I did not intend for that to be part of the forgotten pull request. I've removed the GH action commits, so the PR is now just the improvement to space usage for split images.