microsoft-surface-rt: new device
Adds support for Microsoft Surface RT. Device is based on armv7 SoC and uses EFI, so device package uses grub to boot.
Wiki page https://wiki.postmarketos.org/wiki/Microsoft_Surface_RT_(microsoft-surface-rt)
I added a pointercal calibration file which is needed for touchscreen in osk-sdl, but touchscreen needs also detection fix in tslib or some workaround in osk-sdl or initfs, so touchscreen does not work yet. Unlocking is available only with TouchCover for now.
Grate package is updated to latest git with CONFIG_EFI=y enabled.