imx-codec: add install hooks to fix libraries path

By default, all the libraries are installed under /usr/lib/imx-mm
which causes problems at runtime.

The hooks are inspired from the mechanism used in the Yocto recipe:
https://github.com/Freescale/meta-fsl-arm/blob/krogoth/recipes-multimedia/imx-codec/imx-codec.inc



Tested with the following commands:
 # gst-launch-0.10 audiotestsrc ! mfw_mp3encoder ! fakesink
 # gst-launch-1.0 audiotestsrc ! imxmp3audioenc ! fakesink

[Peter: drop -mindepth/-maxdepth, add comment explaining why this is done]
Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>