Merge #135

135: cargo: Add support for building OpenTitan apps r=alistair23 a=alistair23

This fixes https://github.com/tock/libtock-rs/issues/134 and adds support for building OT apps,

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

Co-authored-by: Alistair Francis <alistair.francis@wdc.com>