utils.bbclass: simplify checksum check, prepare for checksums.ini removal

utils.bbclass: simplify checksum check, prepare for checksums.ini removal

* show note, when there are checksums only in checksums.ini (prepare for
  script for moving all to recipes)
* parse checksums.ini only when there is no checksum in recipe (could be
  faster, but for more checked items in SRC_URI it is parsed repeatedly)
* if one checksum doesn't match then count and show both (md5 as well as
  sha256) - usefull for copy&paste checksums for new recipe.

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>