These have appeared in desktop.mk, regular.mk, vm.mk
over time, and there are two problems around.

The minor one is that mixins have been introduced as
handy reusable bits close in context of their use;
this practically means that they fall under the same
class restrictions as their parent targets, that is
a mixin coming from regular.mk will only be available
for "distro" IMAGE_CLASS, and so on.

The major one is probably the worst design flaw in m-p:
building images from ground up, where ground is a valid
standalone buildable target as well.

Life has shown that we rather want to build up images
the other way around, choosing what essentials go in first
and then fitting the fine details along with the packaging.

The first sign of this difference appeared with ARMv7 Simply:
we had a well-built configuration aiming for x86 ISO, still
we needed roughly the same app/environment configuration
put into armh disk image.

Those platforms were different enough that we didn't actually
plan shipping *lots* of distributions but the problem was clear,
and it was much alike to the one that sprang m-p to life in the
first place (when we had a range of "common" distros and needed
to create and maintain a set of "school" ones that mostly had
similar or even identical difference to their respective base
ones -- and we couldn't do something like conf.d/p8.mk does now).

So mixins are going to become the softer way to turn m-p's
target configuration chain upside down to considerable extent:
build up what you're going to mix into the various deliverables,
and make it as portable across image classes, hardware platforms,
repository branches as feasible so that total maintenance effort
needed goes down or at least doesn't spike too bad.

And here's the first strike at that.