[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
Fix documentation of libbootimg_write_img_* functions
Fix bbootimg's exit codes
Merge pull request #1 from Tasssadar/master

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