lib: os: cbprintf: Interpret unsigned char pointer as string

Static packaging is analyzing string argument type and based
on that builds a package. The most tricky one are pointers
which can be used for %s or %p. %s are strings which may be
transient and must be appended to the package to allow string
formatting from that package when original transient string
location is lost.
It is allowed (gcc printflike extension does not complain) to
using unsigned char for %s. So far static packaging allowed
only char or wchar_t pointers. Extending detection to unsigned
char as well.

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