Support for Arm Cortex-M7 (NUCLEO-F767ZI)

[potsrevennil]

In order to support multi-platform including QEMU, simplify the usage of the tests script and avoid some confusion,
as @mkannwischer suggested, the -e flag is removed, only the platform is required.
Thus the tests interface is changed as follow:

❯ tests func --help
Usage: tests func [OPTIONS] {stm32f4discovery|stm32f767zi|mps2-an386}

Options:
  -v, --verbose             Show verbose output or not
  -i, --iterations INTEGER  Number of tests  [default: 1]
  --help                    Show this message and exit.

• limit the scope of with in nix
• fix multi targets of libopencm3 string concatanation in nix
• support for cortex-m7
• update build ci for m7
• refactor tests script interface by removing the emulate flag
• support for cortex-m7 in the tests script

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)

[mkannwischer]

Thanks @potsrevennil.
Here are some initial comments.
I think the benchmarks need to be reworked a bit because this core actually comes with an instruction cache. We'll likely have to do some cache warming and potentially also have to take the median/average over multiple runs.

[mkannwischer]

Please add emulated tests using the mps2-an500.
I put an example here: https://github.com/mkannwischer/hw/tree/master/mps2-an500.

[mkannwischer]

LGTM now. Thanks!
Please do the mps2-an500 and the -u parameter in a separate PR as tracked by #38 and #34.