RFC: libosmocore generic CRC

Hi, I've just pused in libosmocore a branch sylvain/crc with a proposal for some generic CRC function. The goal here is to provide a base implementation so that channel coding/decoding of the various other projects can use this as a base rather than reimplement their own for each poly. The API is targeted towards that use (using array of ubit_t rather than a buffer of packed bits). The code is generated from a template and expanded into a 8 / 16 / 32 / 64 bits version by the Makefile (each capable of supporting any crc length inferior or equal to that. So a CRC12 would use the 16bits state function). Appropriate doxygen doc is included. If nobody has objection / comments / ... I'll go ahead and merge it. Cheers, Sylvain