scan table file header:
  uint32_t: number of scan tables
  0x10 aligned scan table records

scan table record:
  uint32_t offset in file of table data / 0x200
  uint8_t: length of prefix
  uint8_t's: prefix

table data (0x200 aligned):
  first key down table then key up table
  uint8_t's:
    0x00: bad scancode
    0x01: prefix (go to table)
    0x02: wrong polarity (go to other up or down)
    0x03: good scancode, no keycode
    other: keycode