firmware-kobo-epdc-extractor: extractor for hidden waveform partitions

Draft: common extractor for waveforms on Kobo hidden partitions

Waveforms seems to be specific per device batch and stored on a hidden partition of the sd card. They need to available before framebuffer is available (the corresponding modules are loaded), so it needs to be extracted quite early in initfs. The way to do that is currently quite a hack.