`riscv-rt`: Machine + Supervisor mixed executable

[ZhekaS]

As far as I understand the crates can be compiled with S-mode support, but in that case the assumption will be that it starts and always runs in S-mode.
Is the following use-case supported or any plans for it to be supported:

1. The execution starts in M-mode, certain M-only setup is done (like PMP config etc)
2. The privilege level drops to S-mode for a regular execution flow
3. ECALLs from S-mode are served in M-mode

[romancardenas]

We have no plans for this, but it could be interesting to discuss it. If you like, you could use this issue as an RFC to start developing/shaping how the functionality you propose should look like.