Honestly, the whole idea of forking a sprawling and fast-moving project like buildroot is pretty insane and thus this project is unmaintained.    Buildroot has better facilities for extending their capabilities without having to fork the whole darn thing -- see https://buildroot.org/downloads/manual/manual.html#outside-br-custom

For anyone looking to run buildroot on Raspberry Pis, just use stock buildroot, they have maintained, very minimal Pi based defconfigs.  Currently: 

raspberrypi0_defconfig
raspberrypi0w_defconfig
raspberrypi2_defconfig
raspberrypi3_64_defconfig
raspberrypi3_defconfig
raspberrypi3_qt5we_defconfig
raspberrypi4_64_defconfig
raspberrypi4_defconfig
raspberrypicm4io_64_defconfig
raspberrypicm4io_defconfig
raspberrypi_defconfig
raspberrypizero2w_defconfig

Is this still active?

Maybe you could share the development in it's current state even if it's not finished... to help other people to boost their projects.

hey there

My adventure of embedded system kernel configuration took me here, by now I tried around 20+ kernel compilation but haven't reach a kernel which is booting under 2 seconds without qt5 also

The problem that I facing, I need a rpi3 with very fast boot, but none of any idea on the internet worked for me, What the heck do I wrong man, please give me some advice...

Thanks.

Because its so trimmed down, I assume there is no desktop environment?