galileo: Print elf sections sizes after build

This patch changes Galileo's buildsystem to print the elf sections
sizes after a new image is built. This way we can easily track how
these sections increase or decrease after any change.

To achieve that, we define a custom linking rule which is pretty much
the same as the default linking rule define in Makefile.include, but
we run 'size' command after the image is built.

cpu/x86/Makefile.x86

@@ -7,6 +7,7 @@ CC       = gcc
 LD       = gcc
 AS       = as
 OBJCOPY  = objcopy
+SIZE     = size
 STRIP    = strip
 CFLAGSNO = -Wall -g -I/usr/local/include
 CFLAGS  += $(CFLAGSNO)

platform/galileo/Makefile.customrules-galileo

@@ -6,3 +6,10 @@ OPENOCD_SCRIPTS = $(CONTIKI)/platform/galileo/bsp/openocd-scripts
 debug: $(CONTIKI_PROJECT).$(TARGET)
 	@openocd -s $(OPENOCD_SCRIPTS) -f debug.cfg &> $(shell pwd)/LOG_OPENOCD &
 	@$(GDB) $< -ex "target remote :3333"
+
+CUSTOM_RULE_LINK=1
+%.$(TARGET): %.co $(PROJECT_OBJECTFILES) $(PROJECT_LIBRARIES) contiki-$(TARGET).a
+	$(TRACE_LD)
+	$(Q)$(LD) $(LDFLAGS) $(TARGET_STARTFILES) ${filter-out %.a,$^} \
+	    ${filter %.a,$^} $(TARGET_LIBFILES) -o $@
+	@$(SIZE) $@