comgt: add new script to send ussd request and get the answer

New script for comgt. Should help to fetch balance or any additional information with USSD.
This script uses the standard AT command which should be supported by all modems.

Run-tested on: Mikrotik wAP LTE KIT

Signed-off-by: Kirill Lukonin <klukonin@gmail.com>
[fixed from/sob]
Signed-off-by: Petr Štetiar <ynezz@true.cz>

package/network/utils/comgt/files/ussd.gcom

@@ -0,0 +1,21 @@
+opengt
+ set com 115200n81
+ set comecho off
+ set senddelay 0.02
+ waitquiet 0.2 0.2
+ flash 0.1
+
+:start
+ send "AT+CUSD=1,"
+ send $env("ussd")
+ send ",15"
+ send "^m"
+ waitfor 120 "+CUSD:"
+ if % = -1 goto timeout
+ get 1 "^m" $s
+ print $s
+ exit 0
+
+:timeout
+ print "ERROR: no USSD response, timeout.\n"
+ exit 1