Secure Boot

UEFI can be set to require a trusted chain from boot to execution of OS - aka a Root of Trust. On the Surface RT/2, UEFI is setup to require a trusted chain "secure boot".
This was the main issue for development of other OS's on the Surface RT/2 hardware.
Secure Boot means that we are unable to replace the "bootarm.efi" bootloader with our own bootloader like GRUB as it needs to be signed by Microsoft.
Microsoft is unwilling or unable to provide the key to "unlock" the Surface RT/2 hardware, so we are "stuck" with using their bootloader. There are two possible options to bypass this - Yahallo and Test Certificate bypass.

Test Certificate

A "golden key" bypass which allows us to insert a trusted test certificate into the UEFI Bios allows us to use a signed shim to boot into our own EFI loader (instead of booting into Windows). We still load Microsoft's signed bootarm.efi, which then loads a shim, to load Grub/EFI shell, which can then finally load a Kernel.
Further info on this bootmanager exploit bypass is here

Enabling test signing

The new method

A new, easier method for enabling test signing was discovered. It involves the same techniques, but you only need to boot from a USB twice, you don't even need a windows installed. (An EFI partition could be required, this needs to be verified)
Grab the ZIP file from here, extract it to your USB, and boot your Surface device from it. A menu should show up where you have several options (Install Golden Keys, Uninstall Golden Keys, Apply Yahallo, Undo Yahallo). Select "Install Golden Keys" to enable test signing (You have to use a USB keyboard to select an entry). Your device should reboot and you need to accept a warning.
With this you can also easily disable secure boot with the help of yahallo. Just make sure test signing is enabled and you can select "Apply Yahallo" in the menu to disable secure boot.
You can get further explanation of the tool / USB setup in the development section.

The original method

Note: This only works when you downgraded to Windows RT 8.0 you can brick your device if you use Windows RT 8.1.
Down in this section there is a download for the "golden key" you need to install it, by running InstallPolicy.cmd as admininistrator on your SurfaceRT. It is important that you don't run the file off a USB drive, copy it to your desktop. After running the file you need to reboot, a menu should open which should enable you to enable testsigning. Accept it. After the reboot open up a cmd, again with administrator privilegs and run the following command:
bcdedit /set {default} testsigning on && bcdedit /set {bootmgr} testsigning on
Reboot again, now testsigning should be enabled.

Using test signing to boot files from USB

To use the test signing to boot from USB you need to extract the files from the to the root of a FAT32 formated USB-drive. This setup doesn't include any bootable files, to add one place it into the root directory and name it BOOT.efi. Further reading on how the bootflow works at UEFI Boot Sequence.

Root of Trust

Root of Trust combination with keys and certificates. Using SecureBoot your firmware will check if the operating system you are trying to boot and your bootloader are trusted by you. On each boot-up UEFI firmware will inspect what you are trying to boot and if it's not trusted a security violation will be triggered.
PK: The Platform Key, the master one, the ring to rule them all. The holder of a PK can install a new PK and update the KEK.
KEK: Key Exchange Key is a secondary key used to sign EFI executables directly or a key used to sign the db and dbx databases.
db: The signature databse is a list with all allowed signing certificates or criptografy hashes to allowed binaries. We will use THIS db key to sign our Linux Kernel.
dbx: The dark side of the db. Inverse db. "not-good-db". You name it. It's the list containing all keys that are not allowed.

Further Reading

Secure boot /EFI