Draft: apple-iphone7: Add flash method, package dtb and kernel image

The apple-ipnone7 linux package has to provide additional files for the flasher tool by Corellium. I'm trying to upstream this tool to Alpine Linux aports.

I created a merge request to implement this flasher for Apple devices in pmbootstrap.

The flasher tool works like this:

./load-linux ../../linux-sandcastle/arch/arm64/boot/Image.lzma ../../linux-sandcastle/dtbpack

First argument is a lzma compressed kernel image, the second is a custom dtb package. Both files will be build and packaged with this merge request.