feat: 新增toybox reboot命令

【背景】liteos_a测试套需要reboot命令

【修改方案】
在third_party/toybox/porting新增reboot.c文件，并作自定义修改。
新增reboot命令的支持说明。

re #I3YQ7S
Signed-off-by: yansira <yansira@hotmail.com>
Change-Id: I686a535adbad78935eee04fc283a837a4a7f13f6

apps/toybox/liteos_a_custom.config

@@ -128,7 +128,6 @@ CONFIG_MDEV_CONF=y
 # CONFIG_READAHEAD is not set
 # CONFIG_READLINK is not set
 # CONFIG_REALPATH is not set
-# CONFIG_REBOOT is not set
 # CONFIG_RESET is not set
 # CONFIG_REV is not set
 # CONFIG_RMMOD is not set