drivers: modem: gsm_ppp: Kconfig for autostarting ppp

Autostarting PPP is far from controversial. There are many, many reasons
someone could have for wanting to control exactly when PPP starts. Power
management, NET_EVENT race condition avoidance and any application not
requiring constant and instant use of networking just to name a few.

This commit introduces a Kconfig setting, GSM_PPP_AUTOSTART, which
controls whether gsm_ppp should connect and initialize PPP at boot. It
is set to "y" as default to minimize surprises for legacy code.

Signed-off-by: Benjamin Lindqvist <benjamin.lindqvist@endian.se>