package/weston-imx: split weston-imx off from weston

package/weston-imx: split weston-imx off from weston

Stock weston has been migrated to meson, now the only buildsystem
starting with version 8, but the IMX fork is still stuck With the
older version 6, which only has the autotools buildsystem.

As a consequence, either we make weston a hybrid package that calls
to the appropriate package macro infra, or we split the IMX fork off
to its own package.

meson and autotools have two different ways to express passing options,
so we can't factorise the code paths to support both buildsystems:
we'd need to duplicate those depending on the variant used.

So, it's much more sensible to spin he IMX variant away. Besides, that
will make it easier to drop it, should it eventually gets merged
upstream (or the fork just dies off).

We ensure that the two variants, vanilla and IMX, can't get selected at
the same time, by making weston-imx depend on !weston, and hiding it
behind IMX conditionals.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
[yann.morin.1998@free.fr:
  - drop !BR2_PACKAGE_IMX_GPU_VIV_OUTPUT_WL dependency from stock weston
  - make weston-imx depends on !weston
  - rework commit log
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>