examples/sky-shell now runs out-of-the-box on Z1

For best results, use a Z1 Starter Platform with an sht11 sensor plugged
in the ziglet port.
Keep in mind that the light-sensor is mapped to the potentiometer in this platform.
XXX Tracing why powertrace does not show anything different from 0.

apps/shell/Makefile.shell

@@ -51,3 +51,7 @@ include $(CONTIKI)/apps/collect-view/Makefile.collect-view
 ifeq ($(TARGET),sky)
   shell_src += shell-sky.c shell-exec.c shell-sensortweet.c
 endif
+
+ifeq ($(TARGET),z1)
+  shell_src += shell-sky.c shell-exec.c shell-sensortweet.c
+endif