Custom boot splash screen for Android open source for Shield tablet

How do I replace the current boot splash screen (that appears before the boot animation) with my own? I found bitmaps that look like the splash screen that were extracted from the licensed binaries in /tegra/odm/Ardbeg/nvflash and named tn8_bootsplash.bmp and tn8_bootsplash_land.bmp. I have modified those files, but the build doesn’t seem to include them. When I flash the tablet with the build images, the original boot splash is still shown. I don’t know how they are normally incorporated but it appears that they are not when following the normal build and flashing instructions. Anyone know how to do this?

Make sure you matched the color and bit depth as well as resolution of the original files including alpha channels (or lack thereof in a bmp).