Certain packages are using linux-imx-headers, which enforces them to use
NXP BSP since headers are provided from the NXP kernel tree.
Those recipes, which requires NXP kernel headers, are now including the
use-imx-headers class to make sure the correct headers package is used.
Since the introduction of separation between NXP and Mainline BSP for
certain machines, recipes which are NXP-dependent are getting their
COMPATIBLE_HOST to be marked as (null) effectively making them
incompatible with mainline BSP which is expected behavior.
By extending this restriction into the use-imx-headers class, all
recipes which inherits is (because it is required) are getting
automatically restricted to NXP BSP, hence making package NXP-dependent
much easier.
linux-imx-headers is marked as "NXP-only" but it is done explicitly
inside the recipe in order to avoid confusion of inheriting the class.
