[doc] Add Doxygen Warnings to CI

Doxygen logs lots of warnings to a file. This makes this file available
for download with the other azure logs.

Signed-off-by: Sam Elliott <selliott@lowrisc.org>
diff --git a/azure-pipelines.yml b/azure-pipelines.yml
index aba391b..d8fe256 100644
--- a/azure-pipelines.yml
+++ b/azure-pipelines.yml
@@ -62,6 +62,14 @@
     displayName: Ensure all generated files are clean and up-to-date
   - bash: |
       util/build_docs.py
+      # Upload Doxygen Warnings if Present
+      if [[ -f "build/docs-generated/sw/doxygen_warnings.log" ]]; then
+        echo -n "##vso[task.uploadfile]"
+        echo "${PWD}/build/docs-generated/sw/doxygen_warnings.log"
+        # Doxygen currently generates lots of warnings.
+        # echo -n "##vso[task.issue type=warning]"
+        # echo "Doxygen generated warnings. Use 'util/build_docs.py' to generate warning logfile."
+      fi
     condition: always()
     displayName: Render documentation
   - bash: |
diff --git a/util/build_docs.py b/util/build_docs.py
index 57f0192..67fee99 100755
--- a/util/build_docs.py
+++ b/util/build_docs.py
@@ -241,6 +241,11 @@
     doxygen_sw_path = doxygen_out_path.joinpath("public-api/sw/apis")
     doxygen_sw_path.mkdir(parents=True, exist_ok=True)
 
+    # This is where warnings will be generated
+    doxygen_warnings_path = doxygen_out_path.joinpath("doxygen_warnings.log")
+    if doxygen_warnings_path.exists():
+        doxygen_warnings_path.unlink()
+
     doxygen_args = [
         "doxygen",
         str(SRCTREE_TOP.joinpath("util/doxygen/Doxyfile")),
@@ -255,6 +260,9 @@
 
     logging.info("Generated Software API Documentation (Doxygen)")
 
+    if doxygen_warnings_path.exists():
+        logging.warning("Doxygen Generated Warnings (saved in {})".format(str(doxygen_warnings_path)))
+
     combined_xml = gen_dif_listing.get_combined_xml(doxygen_xml_path)
 
     dif_paths = []