serial: use TERM=vt100 instead of TERM=vt100-nav

serial: use TERM=vt100 instead of TERM=vt100-nav

TERM=vt100-nav was necessary for compat with some ppc hvc devices, a
long time ago. Unfortunately vt100-nav terminfo is not installed by
default on most distros, hence change the default to v100 which is
available universally and still should be a relatively safe and
conservative default.

Should it turn out that vt100 is not really the best choice we can
revert this change again and then ask distros to move vt100-nav into
their default install.