lib: cbprintf: add libc f/printf substitutes

This allows applications that may not use minimal libc avoid the cost
of a second printf-like formatting infrastructure by using printfcb()
instead of printf() for output.  It also helps make sure that the
formatting support (e.g. floats) is consistent between user-directed
output and the logging infrastructure.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>