license.bbclass: Improve parsing time when INCOMPATIBLE_LICENSES is big

The commit 08cbf174 (licenses: Update INCOMPATIBLE_LICENSE for
'or-later' handling) increased the parsing time considerably if there
are many licenses in INCOMPATIBLE_LICENSE. Reorganize the code to get
almost all the time back.

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