https://developer.nvidia.com/linux-tegra-rel-16 Tegra3 aka Surface RT https://developer.nvidia.com/linux-tegra-rel-17 Tegra4 aka Surface RT2
Good for reference DTB's etc https://github.com/OE4T/u-boot-tegra
UBoot has Tegra Mainline support: https://gitlab.denx.de/u-boot/u-boot
The ContextSwitcher switches back from the windows Bootmgr environment back to the UEFI environment. Good for patch reference for SurfaceRT Grub2 display. Also setup for VisualC https://github.com/imbushuo/boot-shim-tegra-3
https://designprincipia.com/virtualize-uefi-on-arm-using-qemu/
A prepatched GRUB2 ready for SurfaceRT or SurfaceRT2. https://github.com/coherixmatts/grub-2.04
I'd also recommend reading Jay Carlson's excellent notes on Embedded Linux here - https://jaycarlson.net/embedded-linux/ as an overview.