cargo_common: use 'config.toml' instead of plain 'config'

cargo_common: use 'config.toml' instead of plain 'config'

cargo configuration has been renamed from plain 'config' to
'config.toml' in rust-1.38.

Using the old name is still supported but creates warnings like

| $ cargo
| warning: `/sdk.../home/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`

Use the new name.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>