The TS-4900 includes u-boot as the bootloader to launch the full operating system. When the i.MX6 processor starts it loads u-boot from the onboard 8MB SPI flash. This allows you to include your boot image on either the SD, eMMC, SATA, NFS, or USB. U-boot is a general purpose bootloader that is capable of booting into common Linux distributions, Android, QNX, or others.
On a normal boot you should see something similar to this:
U-Boot 2013.10-00034-ga8a4e60 (May 02 2014 - 12:19:18) CPU: Freescale i.MX6Q rev1.2 at 792 MHz Reset cause: POR Board: TS-4900 DRAM: 2 GiB MMC: FSL_SDHC: 0, FSL_SDHC: 1 SF: Detected N25Q64 with page size 256 Bytes, erase size 4 KiB, total 8 MiB In: serial Out: serial Err: serial Net: using phy at 7 FEC [PRIME] Hit any key to stop autoboot: 5
By default the board will boot to SD or eMMC depending on the status of CN1_98/MODE2 on startup. On our off the shelf baseboards this pin is brought out as the "SD Boot" jumper. This can be customized further in u-boot as described below.