Add and adjust update script from
https://git.petabyte.dev/petabyteboy/corenix.
The script is meant to be run from the package directory of the
coreboot-toolchain. The script generates a sources.nix files, which
contains all sources used for the coreboot toolchain and some other
tools. Thus, it needs to be stripped down to the necessary sources.
Signed-off-by: Felix Singer <felixsinger@posteo.net>
