[0.2.0] Rewrite the whole library

* Rewrite libbootimg to use "blobs" concept - kernel, initrd,
  second stage and DTB are all objects of same type, which
  enabled me to make functions shorter (they don't have to deal
  with each of them individualy).
* Move image size and config related functions to bbootimg.c - they
  should not be in the library.
* Document all library functions
* Clean-up the API