Add support for PlatformIO platform = native

[delan]

this patch makes cargo-pio compatible with platform = native by making the changes in #96:

• mcu, pio_platform_dir, and pio_framework_dir are now optional in platformio.cargo.py and SconsVariables
• rust_target is now optional in platformio.cargo.py, and completely removed from generated .cargo/config.toml

in my platformio-first project, with these changes i can now call this hello world from both embedded and native:

embedded	native

[ivmarkov]

I am myself a big fan of avoiding the MCU-flash+run step when developing firmware, so yeah - why not having it for Rust+PIO as well?

Just one nit if you could address as per below - i.e. leave the signature of create_config_toml as-is?

Then I'll merge.

[ivmarkov]

Thanks!