LZ4: vendoring in the kernel

We use the approach karkhaz describes in Firefox, also, with some amount of success: each vendored library has an update.sh script which copies the relevant files from a checkout of the relevant upstream version and applies all local patches (usually named after bug numbers that describe the issue and give steps to reproduce). If run, it should produce exactly what is in the tree.

Primarily, it serves as visible documentation so someone doesn't have to remember to run 'git log' on a bunch of files before patching/replacing them. The fact that you have to update it when you patch the vendored code is also a good reminder that someone else (like a future you) will have to deal with that patch on the next code import, and gives good incentive to move patches upstream if possible.