pkg-cmake.mk: globally drive the CMAKE_BUILD_TYPE flag using BR2_ENABLE_DEBUG

This CMake flag is usually used to adjust compiler flags (like: -Ox, -g,
etc).

So, it makes sense for Buildroot to globally drive this CMake flags in
the cmake-package infrastructure.

However, if a package prefer overriding this default setting, it still
can via the <PKG>_CONF_OPT variable.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>