for a quick overview of the boot process, please check this post: https://www.96boards.org/forums/topic/410c-bootloader/. I already provided some details.
The DB410c has proprietary ‘first stage’ bootloaders (before APPSBL) which cannot be rebuilt, or changed. The APPSBL can be rebuilt, there are build instructions in the Linaro Ubuntu release notes, e.g. http://builds.96boards.org/releases/dragonboard410c/linaro/ubuntu/latest/
There is some initial uboot support that someone from the community is doing, see https://www.96boards.org/forums/topic/u-boot-support-for-dragonboard410c/
But even for uboot , it is chain loaded after the LK APPSBL for now.