imagebuilder: fix bundling of DTS sources

imagebuilder: fix bundling of DTS sources

Refer to LINUX_KARCH instead of ARCH when bundling DTS files in the image
builder tarball.

While we're at it, also dereference symbolic links when copying as some
kernel architectures contain symbolic links in their DTS directories.

This fixes aarch64 imagebuilders such as brcm2708/bcm2710 ones in particular
as the kernel refers to "aarch64" as "arm64" internally.

Ref: https://forum.lede-project.org/t/lede-image-builder-problem/3680

Signed-off-by: Jo-Philipp Wich <jo@mein.io>