qcom/venus-4.2: replace split firmware with the mbn file

Follow the example of the sc7280 and sc7180 (venus-5.4, vpu-2.0) and
replace the split firmare files (mdt + bNN) with the squashed mbn file.
For backwards compatibility generate the mdt -> mbn symlink.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>