tools/build: add script to build documentation

This patch adds a doc-build.py script used to build the documentation.
The script can be run within a python virtual environment using the
ModulesVirtualEnv class.
The documentation files are generated under meta-ewaol/public.

Issue-Id: SCM-3032
Signed-off-by: Hugo L'Hostis <hugo.lhostis@arm.com>
Change-Id: I15218cdc797a27c2ab2ce0f4e877e4b0b03abbd8