buildroot

Remove all documentation when BR2_HAVE_DOCUMENTATION is not set

Until now, we had BR2_HAVE_MANPAGES, BR2_HAVE_INFOPAGES and
BR2_HAVE_DOCUMENTATION, but there's no need to have such a
fine-grained control over what documentation gets kept or not.

Therefore, just remove all the documentation (man pages, info pages
and documentation in $(TARGET_DIR)/usr/doc and
$(TARGET_DIR)/usr/share/doc) when BR2_HAVE_DOCUMENTATION is not set.

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