Also add the default values of these macros to the respective
`mpconfigport.h` files, to improve discoverability.
Signed-off-by: Angus Gratton <angus@redyak.com.au>
This makes the stream that the PPP object wraps, which is normally only set
once via the constructor, accessible and configurable via the
`ppp.config()` method.
Signed-off-by: Daniël van de Giessen <daniel@dvdgiessen.nl>