unpackbootimg: support old mkbootimg format extended cmdline as well

mkbootimg: fix off-by-ones to remove enforced null termination

- per osm0sis#8 AOSP's mkbootimg no longer uses null termination at any point in the cmdline or extended cmdline fields allowing the full 512- and 1536-byte lengths, respectively; unpackbootimg already handles this correctly (thanks again @ehem)
- clean up comments/alternate code