minimal: Use printf instead of echo -e.

macOS's echo doesn't have -e option.  printf is in POSIX and more widely
available these days.

Signed-off-by: YAMAMOTO Takashi <yamamoto@midokura.com>

ports/minimal/Makefile

@@ -106,6 +106,6 @@ run:
 	  stty $$saved_
 
 test: $(BUILD)/firmware.elf
-	$(Q)/bin/echo -e "print('hello world!', list(x+1 for x in range(10)), end='eol\\\\n')\\r\\n\\x04" | $(BUILD)/firmware.elf | tail -n2 | grep "^hello world! \\[1, 2, 3, 4, 5, 6, 7, 8, 9, 10\\]eol"
+	$(Q)/usr/bin/printf "print('hello world!', list(x+1 for x in range(10)), end='eol\\\\n')\\r\\n\\004" | $(BUILD)/firmware.elf | tail -n2 | grep "^hello world! \\[1, 2, 3, 4, 5, 6, 7, 8, 9, 10\\]eol"
 
 include $(TOP)/py/mkrules.mk