CircleCI: build on caches, not exponential built workspaces

[tlaurion]

[plan is to make something like] this (n.b. that I'm pretty sure caches were disabled in that) but idea was that instead of using workspaces it would build a cache of each component (so, musl-cross, cb_4.8.1_toolchain, cb_4.11_toolchain, cb_4.13_toolchain and then at some point all the rest of the tools) and then build all the boards in parallel using the caches created.

See Tonux599@39a248b

Along with implementing something like this when we came around to rebuilding, the build would jump straight into building each board with the caches from the full build.

Originally posted by @Tonux599 in #977 (comment)

[tlaurion]

@Tonux599 :
https://gist.github.com/CJBridges/86a5ab831f2763a4ed699061bf902dac from https://discuss.circleci.com/t/skip-job-if-cache-exists/28726/7

might be helpful.
Seems like skipping workspace reusage if cache exists would be one of the most effective optimization?

[tlaurion]

#1015 merged