license.bbclass: canonicalise licenses when dealing with INCOMPATIBLE_LICENSE

license.bbclass: canonicalise licenses when dealing with INCOMPATIBLE_LICENSE

If INCOMPATIBLE_LICENSE=GPL-3.0 but the recipe sets LICENSE=GPLv3, the current
code won't trigger because they're different strings.

Fix this by attempting to canonicalise every license name to a SPDX name, so
both names in this example become GPL-3.0.

[ YOCTO #5622 ]

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