Integrate LZ4 library, compress the ramdisk

lz4/LICENSE

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+This repository uses 2 different licenses :
+- all files in the `lib` directory use a BSD 2-Clause license
+- all other files use a GPL-2.0-or-later license, unless explicitly stated otherwise
+
+Relevant license is reminded at the top of each source file,
+and with presence of COPYING or LICENSE file in associated directories.
+
+This model is selected to emphasize that
+files in the `lib` directory are designed to be included into 3rd party applications,
+while all other files, in `programs`, `tests` or `examples`,
+are intended to be used "as is", as part of their intended scenarios,
+with no intention to support 3rd party integration use cases.