mixer app: bump to latest revision when 4A is enabled

mixer app: bump to latest revision when 4A is enabled

This patch does 2 things:
* pin version eel_5.0.3 when 4A is not enabled
* use AGL_APP_REVISION when 4A is enabled

In the second case, this will pop the following changes
in mixer:
* 62bb1f0 WIP version based on 4a High Level API
* 2aabd6a Add some ignore in .gitignore
* d192c51 Make use of afb-helpers submodule
* e1fe0c6 Add special permission to the mixer
* 21994ea Remove nasty hack to handle HALs
* f35abba make of use of app templates to import right HALs depending on compilation options
* 6bf2ccb make use of alsacore and hal bindings to control audio volume

Change-Id: I5408100aacacb14fe581cf89535bf9b66de0189f
Signed-off-by: Stephane Desneux <stephane.desneux@iot.bzh>