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>

devel/review

@@ -44,7 +44,7 @@ add_reviewed_by() {
       $sob = 1;
 
     } elsif ($sob == 1 and not m/^Signed-off-by:/) {
-      print "Reviewed-by: \n";
+      print "Reviewed-by: $REVIEWER\n";
       $sob = -1;
     }