package/ser2net: S50ser2net: alsoc heck for new config file format

When running ser2net it looks for config files in the legacy conf
format and the new yaml format so we need to allow either in the
sysv init script.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>