i.MX-specific headers are missing in SDK once generated via image
do_populate_sdk task.
This was caused by dropped ALLOW_EMPTY package-controlling variable
which is required for this recipe in order to indicate that package can
have RDEPENDS.
Additional RDEPENDS are also required by packages using new mechanisms
to include linux-imx-headers, this allows headers to be propagated to
the SDK when dependent package is included.
Introduce ALLOW_EMPTY in recipe and RDEPENDS in use-imx-headers bbclass
to have imx headers to be populated in the SDK.
