Tie kernel and rootfs builded separately

Hello,

I have rootfs from Angstrom distribution. It is tar.bz2 archive and it can be probably used on SD card with my Beagleboard. As far as I know, I should unpack it on non-boot partion on my SD card.
Now I want to build kernel downloaded from kernel.org. For kernels I always used uImage.

My question is how it should be done? How kernel should be informed where is rootfs? Should it be in U-Boot or where? Should I copy linux kernel sources to /usr/src/ ?

EDIT:
Is it this parameter in U-Boot: 'root=/dev/mmcblk0p2 rw rootwait'? What then with /usr/src/ directory?