Interesting Repo's

Nvidia Linux files

https://developer.nvidia.com/linux-tegra-rel-16 Tegra3 aka Surface RT https://developer.nvidia.com/linux-tegra-rel-17 Tegra4 aka Surface RT2

UBOOT Tegra

Good for reference DTB's etc

UBoot has Tegra Mainline support:

Context Switcher for Tegra3

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

QEMU for ARM32

Fusee Gelee for T30 (Surface RT)

GRUB BootLoader

A prepatched GRUB2 ready for SurfaceRT or SurfaceRT2.

Misc

I'd also recommend reading Jay Carlson's excellent notes on Embedded Linux here - as an overview.