enable RSSI roaming by default and set it to -75 dBm (#2504)

sd-card/config/config.ini

@@ -106,6 +106,6 @@ LogfilesRetention = 3
 TimeZone = CET-1CEST,M3.5.0,M10.5.0/3
 ;TimeServer = pool.ntp.org
 ;Hostname = undefined
-;RSSIThreshold = 0
+RSSIThreshold = -75
 CPUFrequency = 160
 SetupMode = true