Do not build all books if only www/ is changed

We do not validate the contents of www/ directory and if only files
in it get updated, all the books get build.

Let's do a quick exit for the case of changing only www/.

Change-Id: I05fd5ae58f9351d3d78788f4fe5bbb1e082fb183
Merge "Do not build all books if only www/ is changed"