Cellular: Fix to use PPP stack in PPP mode

[jarvte]

Description

In QUECTEL_BG96 and QUECTEL_BC95 there was missing #ifdefs for
PPP mode and our stack was used instead of PPP stack.

@AriParkkila please review

Pull request type

[X] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

[kjbracey]

If that ifdef is the way the rest of your code is switched, then so be it.

But as a concept I'm not keen on flagging like this. "If the system supports PPP, this driver uses PPP, else it's a command-based driver". Should it not in principle be possible to drive the modem with the AT commands even if the system supports PPP? I would have thought it made more sense to have 2 driver classes - the PPP one and the non-PPP one. A single test image could exercise both...

[ciarmcom]

@jarvte, thank you for your changes.
@ARMmbed/mbed-os-wan @ARMmbed/mbed-os-maintainers please review.

[jarvte]

Question: Ari Parkkila is part of mbed-os-wan but he accepted this before setting mbed-os-wan as a reviewer. Now github shows that mbed-os-wan has not accepted. Is this a github issue or can we do something about this?

[0xc0170]

He was too fast :-) It's OK , We can proceed with CI now

[MarceloSalazar]

@jarvte thanks for the PR! This is important to unblock critical ongoing work.

As a side note, the enablement work on adding Cellular support for a new target (not necessarily inside Mbed OS) is not trivial - this depends on ifdef config inside Mbed OS.
This is very ugly:

mbed-os/features/cellular/framework/common/CellularTargets.h

Lines 23 to 43 in 3b138fb

	#ifndef CELLULAR_DEVICE
	#if defined(TARGET_ADV_WISE_1570) || defined(TARGET_MTB_ADV_WISE_1570)
	#define CELLULAR_DEVICE QUECTEL_BC95
	#elif TARGET_WIO_3G
	#define CELLULAR_DEVICE QUECTEL_UG96
	#elif TARGET_WIO_BG96
	#define CELLULAR_DEVICE QUECTEL_BG96
	#elif TARGET_MTS_DRAGONFLY_F411RE
	#define CELLULAR_DEVICE TELIT_HE910
	#elif TARGET_MTB_MTS_DRAGONFLY
	#define CELLULAR_DEVICE TELIT_HE910
	#elif TARGET_UBLOX_C030
	#if defined(TARGET_UBLOX_C030_N211) || defined(TARGET_UBLOX_C030_R410M)
	#define CELLULAR_DEVICE UBLOX_AT
	#else
	#define CELLULAR_DEVICE UBLOX_PPP
	#endif
	#elif TARGET_UBLOX_C027
	#define CELLULAR_DEVICE UBLOX_PPP
	#elif TARGET_MTS_DRAGONFLY_L471QG
	#define CELLULAR_DEVICE SARA4_PPP

For cellular, I'd like us to consider moving to a configuration based system aligned with other solutions. For example, see connectivity drivers here for ESP8266, using mbed_app.json
https://github.com/ARMmbed/mbed-os/tree/master/components

FYI @kjbracey-arm @sg-

[0xc0170]

@MarceloSalazar Can you create a new issue and mention @ARMmbed/mbed-os-wan for review (will be triaged).

CI started

[AnttiKauppila]

@MarceloSalazar Removing CELLULAR_DEVICE macro is already ongoing: See IOTCELL-1547.

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts