Fix receiving USSD messages not part default GSM character set

RIL delivers USSD string already decoded in UTF-8 format. Hence there is
no need for extra encoding/decoding in rilmodem driver and ofono core.
This implementation will bypass the encoding/decoding mechanism by
using hexadecimal value 0xFF as data coding scheme (reserved SMS and CBS
data coding scheme value in 3GPP TS 23.038)

Signed-off-by: Petri Takalokastari <petri.takalokastari@oss.tieto.com>
[voicecall] Shorten response time when playing single DTMF tone

Signed-off-by: Martti Piirainen <martti.piirainen@oss.tieto.com>
Merge pull request #115 from ptakalok/ussd2

Fix receiving USSD messages not part default GSM character set
Merge pull request #116 from marttipiirainen/dtmf_faster

[voicecall] Shorten response time when playing single DTMF tone
[rilmodem] Implementation for call forwarding activation and deactivation

User can try activate or deactivate the call forwarding from keyboard
instead of registration or erasure. Implementing lacking messages

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Merge pull request #120 from jkangas/forwarding

Request for pulling call forwarding activate and deactivate support to main branch
[ofono] Add MNC length plugin

Signed-off-by: Miia Leinonen <miia.leinonen@oss.tieto.com>