Zephyr support for the C target

[erlingrj]

Adds plug-and-play support for running LF programs ontop of Zephyr RTOS. It has a dependency on Zephyr which can be met by following this simple guide. We require Zephyr version 3.2.0. A proper guide should be written on our webpage.

To compile and emulate a LF program on a Arm Cortex M3 emulation

target C {
  platform: {
    name: Zephyr
    board: qemu_cortex_m3
    flash: true
  }
}

To compile and flash onto a connected nrf52dk board

target C {
  platform: {
    name: Zephyr
    board: nrf52dk_nrf52832
    flash: true
  }
}

The code generator will create, in src-gen a project that can be compiled easily by just cding in and calling west build`.

[arengarajan99]

LGTM, follows the Arduino approach which as long as it passes all the tests should be fine.

[petervdonovan]

It looks like the test failures are mostly just network issues 😢 and not actual failures.

[petervdonovan]

This seems good to me. I left some comments, but none of them are important enough to justify delaying this PR.

[petervdonovan]

There might be a way to change the type of board back to an enum (as it was before this PR), and thereby to avoid having to do this sort of string-matching -- or at least, to restrict the string-matching to a single place that is not buried deep in org.lflang.generator.c. Enums can be constructed with parameters, so even though the total number of boards might be too large to enumerate, you could have a few types of boards (e.g., ARDUINO, QEMU, ...) that also come with a String that specifies the exact name of the board.

[erlingrj]

Good point. @arengarajan99 and I will have to figure out something clever here. Or wait until it is all factored out into the package manager...

[erlingrj]

Good point. @arengarajan99 and I will have to figure out something clever here. Or wait until it is all factored out into the package manager...