codec/mjpeg: rename mjpeg package to jpeg

The mjpeg package is probably inappropriately named; while it was initially written to handle MJPEG stuff, it can also inherently handle JPEG stuff. MJPEG is technically not a codec while JPEG is. With development on the revid timelapse mode, whereby single JPEG images come through at configured intervals, it would be appropriate to make the switch from MJPEG to JPEG.