license.bbclass: canonicalise the licenses named with 'X+'

If INCOMPATIBLE_LICENSE=GPLv3, GPLv3+ should be excluded
as well but not now since there is no SPDXLICENSEMAP for
licenses named with 'X+', we can add all the SPDXLICENSEMAP
settings for licenses named with 'X+' in licenses.conf,
but it's more like a duplication, so improve the canonical_license
function to auto map for 'X+' if SPDXLICENSEMAP for 'X' is
available, so GPLv3+ becomes GPL-3.0+.

(From OE-Core rev: 1d6dab1dbbbfbcb32e58dba3111130157ef2b24f)

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