base/license: Rework INCOMPATIBLE_LICENSE variable handling

This re-writes the INCOMPATIBLE_LICENSE checking code to replace
the WHITELIST_<lic> with
INCOMPATIBLE_LICENSE_EXCEPTIONS = '<pkg>:<lic> <pkg>:<lic> ...'

This initial change leaves most of the code structure in place,
but the code in base.bbclass needs to be re-written to make
the check more consistent around packages (PKGS) and not recipe
names (PN). This also is taking into account the changes for SPDX
licenses.

The aim is to provide a mode consistent variable where the variable
name is known and can easily be queried.

Signed-off-by: Saul Wold <saul.wold@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>