buildroot

package/pkg-cmake.mk: globally disable doc, examples and tests

This patch globally disables doc, examples and tests build for both the
target and host packages.

If needed, these configure flags can be overloaded in the per-package
*_CONF_OPTS variables.

This makes the cmake-package infrastructure even closer to the autotools
one.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>