lib: os: cbprintf: Add flags to CBPRINTF_MUST_RUNTIME_PACKAGE

Add flags to macro which checks if string must be packaged
using runtime approach.

Added flag CBPRINTF_MUST_RUNTIME_PACKAGE_CONST_CHAR. When flag
is set then const char pointers are considered as pointers to
fixed strings and do not require runtime packaging.

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