Added a POC for ALSA/HAL plugin

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+Hal-Plugin 
+
+Object: provide an ALSA a HAL (Hardware Abstraction Layer) to automotive sound cards
+Status: Current version is proof of concept and not a usable product
+Author: Fulup Ar Foll
+Date  : June-2017
+
+HAL-plugin allows:
+ - to expose any existing NUMID under a customized label name, this is order to abstract sound card config.
+ - to add non alsa NUMID supported through a callback mechanism (eg: volume rampdown, power off, ...)
+
+installation
+ - Plugin should be placed where ever alsaplugins are located (typically: /usr/share/alsa-lib)
+ - Callback sharelib directory should be declare in $LD_LIBRARY_PATH
+
+Config
+```
+cat ~/.asoundrc
+  ctl.agl_hal {
+    type  afbhal
+    devid "hw:4"
+    cblib "afbhal_cb_sample.so"
+    ctls {
+        # ctlLabel {numid integer name "Alsa Ctl Name"}
+        MasterSwitch { numid 4 name "My_First_Control" }
+        MasterVol    { numid 5 name "My_Second_Control" }
+        CB_sample    { ctlcb @AfbHalSampleCB name "My_Sample_Callback"} 
+    }
+    pcm.agl_hal {
+        type copy     # Copy PCM
+        slave "hw:4"  # Slave name
+  }
+```
+
+With such a config file
+ - numid=4 from sndcard=hw:4 is renamed "My_First_Control"
+ - numid=4 from sndcard=hw:4 is renamed "My_Second_Control"
+ - numid=4 will call AfbHalSampleCB from afbhal_cb_sample.so
+
+Note: to really implement a usable HAL at minimum every ALSA call should be implemented and read/write of values should be normalised from 0 to 100% with a step of 1.
+
+Conclusion: This demonstrate that implementing a HAL that both abstract ALSA get/set and enable non ALSA support is possible at an acceptable cost
+without having to hack ALSA source code. The beauty of the model is than it is fully transparent to any ALSA application. The limit is that the plugin is loaded
+within every application context, thus interaction with an external event loop remains complete as well as conflict management in case of share resources.