[PATCH */4] add interface property to primary context

[Xu, Martin]

Hi:
"Interface" property of Primary Context is needed by ofono plugin for Connman.
These patches are used to implement "interface" property for Primary Context. Below is the way to implement it.
1.	Create "interface" in Primary Context creating 
2.	 Destroy "interface" in Primary Context destroying
3.	"interface" is created/destroyed by ofono_gprs_context_driver
   a)	"interface" of MBM is inherited from NetworkInterface of modem which is exported from kernel
   b)	Others can use ofono_gprs_context_driver->create/destroy_interface() to create/destroy interface such as ppp0 and tun0.
Please review it.