travis: make verbosity (set -x, make -s) configurable

travis/.travis.yml.example-full

@@ -44,6 +44,7 @@ script:
 # EXTRA    content will be added to make command line
 # STATIC   set to YES for static build (default: NO)
 # TEST     set to NO to skip running the tests (default: YES)
+# VV       set to make build scripts verbose (default: unset)
 
 matrix:
   include: