mesa: make expat dependency a PACKAGECONFIG

mesa: make expat dependency a PACKAGECONFIG

expat is a mesa meson feature which can be disabled so let's make it a
PACKAGECONFIG.

The motivation actually isn't about disabling it (though we now could as
it's only required for a few things) but rather that this dependency is
a non-native one.

The issue is when a native-only (-native recipe name suffix + inherit
native) recipe includes mesa.inc, it'll include the DEPENDS as well but
will then depend on a non-native recipe because BitBake doesn't add a
suffix to dependencies listed in DEPENDS for native-only recipes like it
does for recipes with BBCLASSEXTEND = native. This is a way to
circumvent the issue by making the dependency a PACKAGECONFIG dependency
such that the listed DEPENDS (third parameter) will be properly modified
by BitBake to have the native variant of the recipe pulled in. See[1]
for more info on why this is needed.

Now that expat is a PACKAGECONFIG and that if it's not there expat isn't
autodetected anymore by meson, having xmlconfig PACKAGECONFIG add expat
as a DEPENDS (third parameter) is likely not enough anymore due to expat
feature being explicitly disabled if expat is not in PACKAGECONFIG.
Therefore, remove the third parameter for xmlconfig and add a comment
telling the user there's a PACKAGECONFIG dependency here. Note that
since this commit adds the expat PACKAGECONFIG to all recipes including
mesa.inc, no change in behavior is expected.

This is in preparation for the addition of a mesa-tools-native
native-only package which will include mesa.inc.

[1] https://lore.kernel.org/openembedded-core/07cd1d3c-4d67-4e60-b3d3-75953784f70f@cherry.de/

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>