The dlcache script is useful to avoid unnecessary repeated downloads
of zip archives, similar to the git-cache scripts. The command order
was changed to stay compatible with DOWNLOAD_TO_FILE and the only other
usage of $(DLCACHE) was changed accordingly.
To avoid the risk of collisions, dlcache now uses SHA512 sums instead
of MD5.
If no SHA512 checksum is given to dlcache.sh, it will just act as
$(DOWNLOAD_TO_FILE).
With many open PRs that could benefit from loading SDKs when needed,
instead adding vast amounts of code to RIOTs master, this PR provides
the "functions" `$(DOWNLOAD_TO_STDOUT)`, `$(DOWNLOAD_TO_FILE)`, and
`$(UNZIP_HERE)`.
The first "function" takes one argument, the URL from where to download
the content. It is then piped to stdout. To be used e.g. with `tar xz`.
The second "function" taken two arguments, the destination file name,
and the source URL. If the previous invocation was interrupted, then the
download gets continued, if possible.
The last "function" takes one argument, the source ZIP file. The file
gets extracted into the cwd, so best use this "function" with
`cd $(SOME_WHERE) &&`.
The clumsy name `$(UNZIP_HERE)` is taken because the program "unzip"
takes the environment variable `UNZIP` as the source file, even if
another file name was given on the command line. The rationale for that
is that the hackers of "unzip" hate their users. Also they sacrifice
hamsters to Satan.
