doc: doxygen: add Doxygen predefined macro

A pre-defined Doxygen macro allows for better control of what is
included in the final documentation than maintaining a long list of
CONFIG_* entries.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

doc/zephyr.doxyfile.in

@@ -2229,7 +2229,8 @@ INCLUDE_FILE_PATTERNS  =
 # recursively expanded use the := operator instead of the = operator.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
-PREDEFINED             = CONFIG_ARCH_HAS_CUSTOM_BUSY_WAIT \
+PREDEFINED             = __DOXYGEN__ \
+                         CONFIG_ARCH_HAS_CUSTOM_BUSY_WAIT \
                          CONFIG_ARCH_HAS_CUSTOM_SWAP_TO_MAIN \
                          CONFIG_BT_BREDR \
                          CONFIG_BT_MESH_MODEL_EXTENSIONS \