ci: add envtools support

This commit add envtools suppport to CI to verify if there
is no build issues.

Signed-off-by: Pierre-Jean Texier <pjtexier@koncepto.io>
Acked-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

.travis.yml

@@ -361,6 +361,10 @@ matrix:
     - name: "Build tools-only"
       script:
         - make tools-only_config tools-only -j$(nproc)
+    # Ensure env tools build
+    - name: "Build envtools"
+      script:
+        - make tools-only_config envtools -j$(nproc)
 
     # test/py
     - name: "test/py sandbox"