[API] Add LZ4 for external

[RedGl0w]

Epsilon has LZ4 in its source, but the API doesn't export its functions, so some apps re-implement it externaly (like peanut gb), loosing some ROM...

[zardam]

The rationale behind the API was to have the minimum viable dependency on epsilon to avoid breakages, as it was changing very often. I dont know the exact size of lz4 implementation, but I guess it should be a few kb of flash, and I think not an issue to duplicate.

I'm not very involved anymore in epsilon and external apps for now (not enough time...) sorry, but you can do what you want in Omega.