Add mps2-an385 (Cortex-M3) and mps2-an500 (Cortex-M7) with embed-os

[potsrevennil]

#38

• import mbed-os for emulating with nix
• build mps2-an386 with mbed-os
• add support for mps2-an385 and mps2-an386
• include tests for mps2-an385/mps2-an500 in ci
• cleanup makefile
• add --list-platforms command
• update README
• refactor ci with xargs

• 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]

Two minor issues:

• Please remove the hal/mps2/ directory as it's no longer needed.
• The -v flag does not appear to work for the qemu targets (e.g., tests func mps2-an500 -v).

Rest looks good - thanks @potsrevennil

[mkannwischer]

Works fine for me now. Thank you!