Add new checksum refreshing infrastructure

[staticfloat]

This changes the contrib/refresh_bb_tarballs.sh into a
contrib/refresh_checksums.mk which can be run in parallel much
faster than the original script thanks to the new checksum-foo targets
for all checksummable targets.

This will allow us to build a CI bot that automatically does this for
all PRs, hopefully noticing cases where the checksums are
missing/extraneous ones have not been pruned.

[staticfloat]

The proposed CI bot will do, essentially,

rm -rf deps/checksums
make -f contrib/refresh_checksums.mk -j${NPROC}
[ -z $(git status --porcelain) ]

The happy path is that the checksums all get recreated instantly. However, if some checksums fail to be recreated, this means the user should have pruned those checksums, or if there are new checksums, the user should have added those checksums. In this manner, I hope to avoid Fredrik having to post another Bernie Sanders meme at me. ;)

[vtjnash]

Wouldn't this be easier as a direct bash script, without the Makefile generator, since you haven't actually defined any real targets? 😂

[StefanKarpinski]

I've been thinking for a while that we should take a lot of the intense functionality that's crammed into our makefiles and turn them into plain old scripts instead and simplify the makefiles. Of course, I also think we should stop supporting in-tree builds of our deps and only support BB or system builds with a separate repo for building our own copies of all the deps from source.

[staticfloat]

Wouldn't this be easier as a direct bash script, without the Makefile generator, since you haven't actually defined any real targets? 😂

Ironically, I started with a direct bash script, but the parallelism here is so important that I ended up just using make so that I can easily -j this. I was halfway to building my own semaphore/FIFO-based job queuing system based on this SO when I stopped myself and just used make.