UIC-3286: fix cargo warning about config file

warning: `.cargo/config` is deprecated in favor of `config.toml`
note: if you need to support cargo 1.38 or earlier, you can symlink `config` to `config.toml`

Forwarded: SiliconLabsSoftware#22
Signed-off-by: Philippe Coval <philippe.coval@silabs.com>

Author: Luisky