U-boot is a bootloader. A Linux kernel is completely separate from a bootloader. Typically, the bootloader (U-Boot, in your case) launches a Linux kernel, and may optionally set up a RAM based root filesystem (initrd) to be used by the kernel. Is the file you describe as rootfs.ext2.gz.uboot
the Linux initrd?
If so, I will guess that you want to know either
- How to cross compile applications for your target architecture
- How to roll cross-built applications into an existing initrd root filesystem
- Both of the above