package/cmake: with BR2_ENABLE_DEBUG use RelWithDebInfo

>From the documentation on BR2_ENABLE_DEBUG, the intention is to get
a build with debug symbols and not a "debug build" since that can have
the unintended consequence of being a different code path then a
release build type definition.

Switch the "Debug" to "RelWithDebInfo" in the cmake package support
to accomodate getting the debug symbols and still be a release
build.

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