In the BSP, Buildroot creates a root file system (output/buildroot-2014.05/output/target) and also packages it up as a squashfs.
Then, you use the BSP command:
./build.sh axfs
which will take the output files from Buildroot (output/buildroot-2014.05/output/target) and create an AXFS image.
That generated AXFS image will be located here:
output/axfs/rootfs.axfs.bin
So, after you add packages to Buildroot (./build.sh buildroot menuconfig) and rebuild your file system (./build.sh buildroot), you need to run the axfs command (./build.sh axfs) to make the equivalent AXFS image to program.
Then, you can use the "./build.sh jlink" command to download the new image to the board (u-boot) for re-programming.
Chris