lib: os: cbprintf: Add option to enable pointer checking

Add CONFIG_CBPRINTF_CONVERT_CHECK_PTR which enables support for
checking if string candidate pointer is not %p. It is by default
disabled when logging strings are removed from the binary. Option
is added to save code.

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