directfb: add host-directfb target for directfb-csource

directfb-csource is a code generation tool that may be needed to build
other packages. Add host-directfb target that installs directfb-csource
in the host_dir.

Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>