classes/sanity: fix handling of bblayers.conf updating

classes/sanity: fix handling of bblayers.conf updating

Fix the fairly long-standing problem of treating a newer bblayers.conf
in the same manner as an older one (reporting that it had been updated
even if nothing was done). The recent work to do a reparse without
having to manually re-run bitbake turned this from an annoyance into an
endless loop, so it had to be fixed.

As part of fixing this the following changes have been made:
* Extensions are now implemented using a function list, so distro layers
  can add their own functions which should either succeed (indicating
  they have successfully updated the file) or raise an exception
  (indicating nothing could be done). The functions are called in
  succession until one succeeds, at which point we reparse.
* If we can't do the update, the error message now says "older/newer"
  instead of just "older" since we only know the version is different.

(From OE-Core rev: 46b00fdfc9d1e3dc180de087bae2682a1baa2954)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>