audio/cmake: add more inline documentation

The audio top-level CMakeLists.txt is a bit complex to follow
as it has two separate set of build rules. One to build normal
SOF firmware target, with rules in subfolders and another
for the host-library build (not using subfolder CMakeLists.txt).
The two sets are broken with a return() in the middle.

Add inline commentary to the different sections making this
easier to follow.

Link: #8606
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>

Author: kv2019i

src/audio/CMakeLists.txt

@@ -39,6 +39,8 @@ if(it_is) ###  Zephyr ###
 	return()
 endif()
 
+### XTOS normal build (not shared library) ###
+
 if(NOT CONFIG_COMP_MODULE_SHARED_LIBRARY_BUILD)
 	add_local_sources(sof
 		host-legacy.c
@@ -143,6 +145,8 @@ if(NOT CONFIG_COMP_MODULE_SHARED_LIBRARY_BUILD)
 	return()
 endif()
 
+### Shared library build (CONFIG_COMP_MODULE_SHARED_LIBRARY_BUILD) ###
+
 subdirs(pipeline)
 
 if(CONFIG_COMP_MODULE_ADAPTER)