doxygen: new package

Host package only.

[Thomas:
 - add missing dependencies on host-flex and host-bison
 - add $(HOST_MAKE_ENV) to the configure, build and install steps, to
   make sure that the doxygen build system can find flex and bison.]

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>