package/cdrkit: fix namespace of variables

Not all cdrkit variables are prefixed with CDRKIT_, so they leak.

Most importantly, they look like they belong to another package's
namespace, cmake, as they start with CMAKE_ (but fortunately, they
have no impact on cmake, as they are not used by our infras.)

Fix that by removing the intermediate variables, and directly set
the CDRKIT_CONF_OPT variable.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>