Platformio-core: Add support for rust

[abelykh0]

Add support for rust and cortex-m-rtfm
https://github.com/japaric/cortex-m-rtfm

[ivankravets]

Related links:

• https://community.platformio.org/t/how-would-i-implement-a-rust-compiler-that-links-with-any-framework/14113
• https://github.com/dzervas/platformio-arduino-rust

[dzervas]

I've created a POC on the above linked repo, take a look. Shouldn't be too hard to use my example for another MCU