license_image.bbclass: Rename license-incompatible to license-exception

license_image.bbclass: Rename license-incompatible to license-exception

There is currently both an incompatible-license and a
license-incompatible QA message. This is very confusing.
However, license-incompatible is only used to output a message when a
package is included in an image despite it having a license that is
normally incompatible (by using the INCOMPATIBLE_LICENSE_EXCEPTIONS
variable). To better match how it is used and to distinguish it from
incompatible-license, rename it to license-exception.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>