Reproducible builds

[gendx]

As mentioned in tock/tock#1666, binaries built with Rust are not reproducible by default. One main reason is panic messages that embed an absolute path to the file containing them (so building with --panic-console isn't reproducible by default).

We should apply the same techniques as in tock/tock#1666 to make OpenSK builds reproducible.

This would avoid issues like spurious linker errors when using a long build path, as discovered in #67.

[gendx]

Re-opening to remember to update the kernel once tock/tock#1668 is merged upstream.

It could also be worth displaying a hash of the binaries like in tock/tock#1669.

[gendx]

As mentioned in #80 (comment), the crypto material is generated for each build (as intended). We could have a workflow that uses fixed material to test for reproducibility.