riot-rs-embassy: improve SWI selection

[kaspar030]

This PR tries to make the incoming wave of different SWI choices more managable.

• add executor_swi!(NAME)-macro (reduces code duplication, see e.g., arch/nrf/mod.rs)

• adds logic to build.rs that, if set, generates a swi.rs that basically just contains executor_swi!(NAME) with NAME taken from the env variable CONFIG_SWI. This will be used in add stm32 support #237.

[kaspar030]

generates a swi.rs that basically just contains executor_swi!(NAME) with NAME taken from the env variable CONFIG_SWI.

I didn't find a way to do this directly in Rust. There might be one using proc macros, but I dug quite far, IMO the build.rs solution is preferable. The difficulty lies in env!() spitting out a string literal, but we want to use the value in a use foo; statement.

[chrysn]

Looks like a good simplification; see comment inline.

I'm not super happy with how (as I've learned as part of reviewing this) InterruptExecutor::start as a safe function trusts the user to pass in the right interrupt, but that's nothing to fix in this PR.

[ROMemories]

This allows arbitrary Rust injection from the environment; since it's only expecting an identifier, I think it should be validated by checking that the environment variable value is only composed of a-z, A-Z, 0-9, and _.

[chrysn]

Generally, an attacker in control of the execution environment already owns the build process and host machine.

If we want to introduce a check, syn::Ident can be parsed from a string without making arbitrary validity choices.