doxygen: reference requirements

Add special command @req to reference requirements. This will help with
traceability between tests and requirements using doxygen.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/doc/zephyr.doxyfile b/doc/zephyr.doxyfile
index 9579745..1348995 100644
--- a/doc/zephyr.doxyfile
+++ b/doc/zephyr.doxyfile
@@ -227,6 +227,8 @@
                          "r=\verbatim embed:rst:leading-asterisk" \
                          "er=\endverbatim"
 
+ALIASES 	       += "req=\xrefitem req \"Requirement\" \"Requirements\" "
+
 # This tag can be used to specify a number of word-keyword mappings (TCL only).
 # A mapping has the form "name=value". For example adding "class=itcl::class"
 # will allow you to use the command class in the itcl::class meaning.