travis: build HTML docs

Several patches delivered incorrect restructured text as documentation.
We should be able to discover this in Travis CI.

Provide a build step for 'make htmldocs'.

Add required package graphviz.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

.travis.yml

@@ -39,6 +39,7 @@ addons:
     - libisl15
     - clang-7
     - srecord
+    - graphviz
 
 install:
  # Clone uboot-test-hooks
@@ -353,6 +354,10 @@ matrix:
     - name: "cppcheck"
       script:
         - cppcheck --force --quiet --inline-suppr .
+    # build HTML documentation
+    - name: "htmldocs"
+      script:
+        - make htmldocs
     # search for TODO within source tree
     - name: "grep TODO"
       script: