sanity.bbclass: bblayers.conf should be updated automatically

sanity.bbclass: bblayers.conf should be updated automatically

- add check_bblayers_conf bitbake function which does the bblayers.conf
v4 -> v5 update if necessary (every layer should make its specific
bblayers.conf upgrades appending to the check_bblayers_conf function)
- we ask the user to re-run bitbake because we can't trigger reparsing
without being invasive

[YOCTO #3082]

(From OE-Core rev: 03ad4edace5db9c6e15ca776d06d20b7d4e42afc)

Signed-off-by: Constantin Musca <constantinx.musca@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>