devel/review: take default reviewer from environment

When adding a 'Reviewed-by'-line, also add the reviewer
passed through the REVIEWER environment variable. In that
way, adding the reviewer for patch series all reviewed
by the same person gets easier.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>