pine64-pinephone: Add support for eMMC Vccq hardware mod

Related issue: #1285 (closed)

This merge request contains the following changes:

• Compile pinephone dts with -@ option so that is contains symbols used by DT overlays
• Make U-boot script boot.scr load an external user.scr script, no effect if user.scr is not present
• In user.scr load and apply the Device Tree Overlay that matches the regulator change of Vccq mod and enables HS200
• Ship user.scr and pinephone-vccq-mod.dtbo files in a subpackage of device-pine64-pinephone, that can be installed manually.

This change has no effect on a regular PinePhone pmos installation.
If one performs the eMMC Vccq mod the phone will boot fine but without speed increase.
By installing the package device-pine64-pinephone-vccq-mod, a Device Tree Overlay is applied at boot and you get double speed from the eMMC (HS200 mode).

The change is persistent for the current pmos installation, and should survive u-boot and kernel updates, but the subpackage has to be installed again after formatting.

I have tested the changes on my modded PinePhone 1.2, it works fine with and without the subpackage installed.

I can provide a guide for the hardware modification in the wiki, pinephone page or dedicated page?.