Makefile.contiki: Added -Os to remove undefined symbol

handle_periodic_timer() in rpl-timers.c relies on the optimizer to
remove the if(0)-guarded call to rpl_ns_periodic() when storing
mode is selected. This does not happen with clang's default options
and thus leads to an error for the undefined reference at link time.

This modification adds -Os to fix this.

examples/contiki/Makefile.contiki

@@ -23,7 +23,7 @@ endif
 
 all: server
 
-CFLAGS += -ffunction-sections
+CFLAGS += -Os -ffunction-sections
 LDFLAGS += -Wl,--gc-sections,--undefined=_reset_vector__,--undefined=InterruptVectors,--undefined=_copy_data_init__,--undefined=_clear_bss_init__,--undefined=_end_of_init__
 
 CFLAGS += #-DSHORT_ERROR_RESPONSE -DNDEBUG