testing/device-qcom-msm8916: New port

This MR adds a generic port for most msm8916 devices.

This port is experimental and is made to have a minimal base for us to test later changes that aim at generic images, as well as to be able to asses current problems better.

The generic port has two modes:

extlinux.conf

"Future" lk2nd has gained support for automatic dtb selection and uses extlinux.conf as it's bootloader configuration. This port makes use of that feature and generates a generic image that supplies all relevant dtb files for lk2nd to pick from.

EFI

Thanks to amazing work @calebccff did in U-Boot, msm8916 devices can run close-to-mainine U-Boot to make use of its EFI runtime. This port enables EFI support in msm8916 kernel and makes use of systemd-boot as EFI bootloader. The DTB handling is opaque to the OS in this case and is handled out-of-band. (msm8916-mainline/gen-uboot-img is used to generate a bootchain package that includes lk2nd and recent dtb files that are packaged into a single image with u-boot.)

Cc: @Minecrell