s6-portable-utils: new package

This new package provides s6-portable-utils, a set of tiny general Unix
utilities, often performing well-known tasks such as cut and grep, but
optimized for simplicity and small size.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>