openconnect: support reading password from script

"token_mode" add support for "script", which execute "token_script" to
get the password.  Some token is not supported by OpenConnect natively,
e.g. "MobilePass" or "Softoken II" used in Cisco VPN

Signed-off-by: Gavin Ni <gisngy@gmail.com>
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

net/openconnect/README

@@ -26,6 +26,10 @@ config interface 'MYVPN'
         #option token_mode 'hotp'
         #option token_secret '00'
 
+	# tokens from script
+	#option token_mode 'script'
+	#option token_script '/lib/custom/getocpass.sh'
+
 	# Juniper vpn support
 	#option juniper '1'