build: don't try debugserver if there's no DEBUG_SCRIPT

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
diff --git a/Makefile.inc b/Makefile.inc
index e034dd6..d4ec795 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -99,8 +99,14 @@
 export BOARD_NAME
 endif
 
+ifneq ($(DEBUG_SCRIPT),)
 debugserver: FORCE
 	$(Q)$(CONFIG_SHELL) $(ZEPHYR_BASE)/scripts/support/$(DEBUG_SCRIPT) debugserver
+else
+debugserver: FORCE
+	@echo Debugging not supported with this board.
+	@echo Please check the documentation for alternate instructions.
+endif
 
 initconfig: $(DOTCONFIG)