busybox: migrate supporting files required to generate ramdisk

Instead of depending on a separate directory outside of build-scripts
to hold the supporting files required to create the cpio image, migrate
those supporting files into build-scripts. This helps with removing
the dependency on requiring a separate repo to host these supporting
files.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I6ea1b5ea6cab4251c4545a33d818527d3d3f0e40