bitbake docs: use dblatex to build the pdf bitbake manual

bitbake docs: use dblatex to build the pdf bitbake manual

Fix [BUGID #593]

The current manual build fails for printing formats which use latex as an
intermediate format. This bug has been reported in multiple locations and I
haven't found a solution posted to any of them.

Using --with-dblatex uses dblatex to make the conversion and successfully
generates the pdf. It adds a dependency on dblatex and its dependencies.

(From Poky rev: b26640f30a4c80fcedda0d19bcf96ed412062a33)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
CC: Joshua Lock <josh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>