lib: os: cbprintf: Renamed flags used for conversion function

At some point, package copy function was extended and renamed
to cbprintf_package_convert. However, flags used by this
function were not renamed and used contained COPY idiom.
Deprecating flags with COPY and replacing them with flags
with CONVERT idiom to match function which is utilizing them.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>