lib: cbprintf: refactor for upcoming enhancement

The extraction of values from varargs was inlined in cbvprintf()
because va_list arguments cannot be passed into subroutines by
reference, and there was only one place that needed this
functionality.  An upcoming enhancement requires extracting the
arguments but processing them later, so outline the extraction code.

Signed-off-by: Peter Bigot <[email protected]>

Author: pabigot