To create your own rootfs with Buildroot, clone the Buildroot Repository from Github. Make sure to checkout a release version of buildroot and not the master branch!

Then configure buildroot according to your needs. Configuration can be done typing the following command in the root folder of buildroot:

$ make menuconfig

For the Colibri iMX6 you can find a buildroot defconfig in our NTB Buildroot-Fork: https://github.com/ntb-ch/buildroot/tree/cb20