Hi Olli
i.MX6UL has ROM (it can not be reflashed by user) which is used for booting
and described in Chapter 8 System Boot i.MX6UL Reference Manual
http://www.nxp.com/docs/en/reference-manual/IMX6ULRM.pdf
Additional bootloaders, like uboot can be flashed to external boot devices like emmc, nand, e.t.c.
Procedures are described in documentation provided on link
i.MX Software|NXP
Best regards
igor
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