pppd: Add an option to not to overwrite /etc/resolv.conf

pppd: Add an option to not to overwrite /etc/resolv.conf

By default pppd built by buildroot writes the list of nameservers to
/etc/resolv.conf instead of /etc/ppp/resolv.conf
This is not the default behavior of pppd and breaks name resolution
if several network interfaces are used. The change makes this optional
and enabled by default, to be backward compatible and to add a possibility
to turn this behavior off, if required.

Signed-off-by: Maksim Salau <msalau@iotecha.com>
[Arnout: extend the help text to explain that it won't work on readonly
 rootfs]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>