Are you designing a board or working with an existing design? Usually, you will need your bootloader to run from NOR flash. The uBoot bootloader can read USB and filesystems in NAND flash. You can google uBoot and find a lot of information about it. dBug is available for download from Freescale. dBug cannot boot from USB or NAND flash so you have to have your kernel in NOR flash and copy to RAM. Once booted uClinux can access USB and JFFS2 on NAND.
