I have download the buildroot ,Can I use it create rootfs and cross complie ?

Yes.

Buildroot is a preconfigured collection of scripts which downloads, configures, and builds cross compilers, the kernel, busybox, and most of the other stuff needed for a simple, small complete linux distribution and root file system.

It is relatively easy to go from there to adding your own programs and such to it so your entire build process is automated.

-Adam

And the buildroot script I provide also builds the U-Boot
and the at91-bootstrap.