package/pkg-cmake.mk: move CMAKE_BUILD_TYPE definition into toolchainfile.cmake

The chosen CMAKE_BUILD_TYPE encodes an option of the Buildroot
configuration, so it makes more sense to save it in the
toolchainfile.cmake than to pass it during configure.

It is still possible to override the build type on the cmake
command line.

Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas: reword description in the CHANGES file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>