Finish kernel support in Go update payload implementation

[marineam]

Dug this code out of an old branch I left unfinished last summer and cleaned it up.

PCR policy data is left unimplemented but we haven't started using that feature anyway.

Verified by switching the generator to use bzip2 instead of lbzip2 and diffing the protobuf manifest generated against what update_engine's delta_generator does, resulting in:

+++ out.txt	2017-05-24 13:09:18.025764192 -0700
@@ -10932,7 +10932,7 @@
     data_length: 873139
     dst_extents: <
       start_block: 8448
-      num_blocks: 256
+      num_blocks: 233
     >
     data_sha256_hash: "\r\332\216p\376\006\335\201\265L\320\237\023\001\246+\025\376\3451\274?\240\213\026\177\377\346\375\311&~"
   >

That 256 value is from a known but harmless bug in delta_generator. If it wasn't for that the two payloads would be bit-for-bit identical.