images: add microsoft-surface-rt

This device is in testing category, but I'm going to propose moving it to community very soon.

The feature table implies very good support, but there is an issue with performance because of missing support for L2 cache and DVFS which makes performance on pmOS very poor (however this is about to improve as well!).

Meanwhile, stock OS of this device is not able to perform almost anything, it is not even able to run a modern browser like chromium or firefox, which makes many users to look for alternative options for this device. Some people find out about pmOS but struggle because of missing pre-built images.

There is also a very good thing about having prebuilt images on this device: there are 7-zip and Rufus utilities for Windows RT which makes it possible to flash pmOS to USB storage from the stock OS, so users don't need an additional PC to install pmOS on this device. With pmbootstrap it is not possible.

Thus, I suggest to enable pre-built images for it

wiki: https://wiki.postmarketos.org/wiki/Microsoft_Surface_RT_(microsoft-surface-rt)