devtool: add license checksum change handling to 'devtool upgrade'

devtool: add license checksum change handling to 'devtool upgrade'

Specifically, 'devtool upgrade' will now do these things:

1) determine if any of the license checksums need updating; if so,
write the new checksums into the LIC_FILES_CHKSUM value in the recipe
that is written to the workspace;

2) print a notice to the standard output:

NOTE: New recipe is /home/ak/development/poky/build/workspace/recipes/puzzles/puzzles_git.bb
NOTE: License checksums have been updated in the new recipe; please refer to it for the difference between the old and the new license texts.

3) and the cool part: devtool will create a diff of the old and new licenses,
and write the diff into the workspace recipe as a comment, like this:

======
 FIXME: the LIC_FILES_CHKSUM values have been updated by 'devtool upgrade'.
 The following is the difference between the old and the new license text.
 Please update the LICENSE value if needed, and summarize the changes in
 the commit message via 'License-checksum-change:' tag.
 (example: 'License-checksum-change: copyright years updated.')

 The changes:

 --- LICENCE
 +++ LICENCE
 @@ -1,8 +1,9 @@
  This software is copyright (c) 2004-2014 Simon Tatham.

  Portions copyright Richard Boulton, James Harvey, Mike Pinna, Jonas
 -Klker, Dariusz Olszewski, Michael Schierl, Lambros Lambrou, Bernd
 -Schmidt, Steffen Bauer, Lennard Sprong and Rogier Goossens.
 +Kölker, Dariusz Olszewski, Michael Schierl, Lambros Lambrou, Bernd
 +Schmidt, Steffen Bauer, Lennard Sprong, Rogier Goossens and Michael
 +Quevillon.

  Permission is hereby granted, free of charge, to any person
  obtaining a copy of this software and associated documentation files

======

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>