examples: Add a hello_world
examples: Add a hello_world

Add a really small "Hello World" example. This is useful for testing
libtock-rs on the HiFive1 as the app is small, simple and easy to debug.

Signed-off-by: Alistair Francis <52ecf00b70a50079ae8d5fa09b9af6904950b63c@wdc.com>

tock/libtock-rs