lib/cbprintf: CONFIG_CBPRINTF_LIBC_SUBSTS should be optional

When aiming for the smallest build result, it is desirable to
use CONFIG_CBPRINTF_NANO together with CONFIG_MINIMAL_LIBC. However
that doesn't mean we need all the functions enabled by having
CONFIG_CBPRINTF_LIBC_SUBSTS=y which increases the binary size even if
those functions are not used and not linked in.

When CONFIG_CBPRINTF_LIBC_SUBSTS=n such functions are not defined
anyway so z_cbvprintf_impl() won't be invoked with missing
functionalities.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>