The Intel Atom CE4100 used in the Boxee Box supports directly booting off the NAND flash; the first stage bootloader is presumably in ROM.
Stage0: ROM-based loader that loads the first block of NAND flash and grabs some header values
Stage1: Flash-based loader in the second block group (block 8-15) that loads and verifies the bootloader
Stage2: Bootloader — presumably Intel CEFDK Bootloader — loads a kernel and initrd from flash, and boots it
Stage4: squashfs filesystem with userspace