Working on ISO15693 support for librfid

It's really been bugging me for a long time that librfid was lacking support for the ISO15693 protocol. The supported reader hardware ASIC can do it, but librfid always was lacking the respective code. It has been on my agenda even three years ago, but there were always higher priority items to pre-empt it.

In December 2007, Bjoern Riemer submitted a long patch to add partial ISO15693 support to librfid. The size of the patch reflected the huge amount of work that must have went in that code. So I couldn't really afford to let all that work bit-rot. I went through several iterations of code cleanup, starting with cosmetic issues, and digging deeper and deeper. I think it now doesn't really look all that similar to what Bjoern originally did, but at least now we have a working and fairly well-organized ISO15693 anti-collision implementation in librfid.

However, ISO15693 has many different options with regard to speed, modulation, coding, etc. All those combinations have to be carefully tested. What's also missing is a way how to iteratively cycle through all available ISO15693 tags within range, similar to what we do with ISO14443A and B.

Once that work has been finished, the actual higher-level standard commands, as well as the nxp I*Code2 and TI Tag-it vendor-specific extensions can be implemented on top. This can probably be done on one or two more days of additional work. Stay tuned...