Merge #246

246: tock: Bump to the 1.6 release r=jrvanwhy a=alistair23

Bump to the latest release. This now uses a long lasting OpenTitan boot
ROM to avoid CI failures, see https://github.com/tock/tock/pull/2170

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

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