doc: provide a target to only generate doxygen

Sometimes you are only interested in doxygen, so need to wait 10 minutes
for everything to generate in this case. Now just do:

 make doxygen

and get only the doxygen output.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

Author: nashif

Makefile

@@ -19,3 +19,6 @@ htmldocs-fast:
 
 pdfdocs:
 	mkdir -p ${BUILDDIR} && cmake -GNinja -DDOC_TAG=${DOC_TAG} -DSPHINXOPTS=${SPHINXOPTS} -B${BUILDDIR} -Hdoc/ && ninja -C ${BUILDDIR} pdfdocs
+
+doxygen:
+	mkdir -p ${BUILDDIR} && cmake -GNinja -DDOC_TAG=${DOC_TAG} -DSPHINXOPTS=${SPHINXOPTS} -B${BUILDDIR} -Hdoc/ && ninja -C ${BUILDDIR} doxygen

doc/CMakeLists.txt

@@ -347,6 +347,12 @@ add_custom_target(
 )
 add_dependencies(htmldocs html)
 
+
+add_custom_target(
+  doxygen
+)
+add_dependencies(doxygen doxy_real_modified_times)
+
 add_dependencies(latex content doxy_real_modified_times kconfig)
 
 add_custom_target(