Buildroot：在Linux登录之前/之后启动一个程序，并在X.org启动之后启动

I would like to create a very small linux that boots up and loads a program into execution. I am using Buildroot in this endeavor. It would be nice if I could install the X.org without any desktop management environment (like Gnome or the many others) and just run a GUI app that would display various functionalities. I'd like to use it for accessing HDDs and USB sticks, to copy data from a partition to another.. simple tasks.. like this one.I would like to create a very small linux that