EFR32MG24 support

[puddly]

This PR ports Ziggurat to the EFR32MG24. Silicon Labs doesn't really support Rust in any way so this requires a bit of trickery: we pull in the Simplicity SDK (automatically via build.rs) and compile a tiny shim library in order to use RAIL, the hardware accelerated AES APIs, and access the C/C++ configuration #defines. Otherwise, we'd need to rewrite a sizeable portion of the SDK.

This build targets the ZBT-2 as a development platform for its exposed debugging headers but can easily be retargeted to run on the cheap SILabs MG24 dev kit as well, just make sure to overwrite the config/ files with project output from Simplicity Studio or https://github.com/NabuCasa/silabs-firmware-builder.

The earlier ESP32 port took care of all of the resource management issues so this is essentially just hooking up a UART, radio RX/TX, and hardware accelerated AES.

A few TODOs:

• Energy scanning.
• Finish 802.15.4 ACK frame pending bit support. This can be left enabled permanently but the other radio configurations dynamically set it based on enqueued frames (child devices and new joins) so we can do it here too.
• Clean up interrupt handling.