It's present in the **Boot ROM**, not a partition in the eMMC.

Source: **[MediaTek details: SoC startup][2]**

The Boot ROM will be embedded inside the processor chip. ([read more][1])

Further Technical details on working of Preloader: **[MediaTek details: Partitions and Preloader][3]**


  [1]: http://stackoverflow.com/questions/15665052/what-is-the-difference-between-a-bootrom-vs-bootloader-on-arm-systems
  [2]: https://sturmflut.github.io/mediatek/2015/07/02/mediatek-details-soc-startup/
  [3]: https://sturmflut.github.io/mediatek/2015/07/04/mediatek-details-partitions-and-preloader/