What I know about MLO is the following (worked on smart phones - TI OMAP4 (namely OMAP 4430) for a year in Y2010/Y2011). Here it is:

My best understanding is that x-loader (MLO) is nothing more then minimalistic bootloader, derived from u-boot. In my time it was around 35KB of size, but I see that it grew with the time (over 64KB). It's main task is to specifically configure certain SoC I/O pins. AAs I remembered, at the time it needed to also configure SoC's DRAM (the last SoC's layer), since u-boot requires at least 256KB. Also, the security is added to MLO, so it must be signed (this came as a feature in Y2011).

Zoran