kernel.bbclass: Give sanity check function an opt-out variable

kernel.bbclass: Give sanity check function an opt-out variable

Having no opt-out method and adding the task to linux-yocto.inc was
causing issues. For example, linux-yocto-dev would often fail because
it uses AUTOREV with no way to dynamically change the PV.

Add a variable to turn off the sanity check, allowing an easy opt out,
and set the opt-out variable in linux-yocto-dev, fixing the issue with
AUTOREV.

(From OE-Core rev: 1b6a3c17874ead7ee0957e67329aa3bd019fa129)

Signed-off-by: California Sullivan <california.l.sullivan@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>