This adds libofonobinderpluginext.so library which defines interfaces
and provides the mechanism for registering extensions.
The only interface defined so far is BinderExtImsSmsInterface
The logic is based on ofono-ril-plugin. Only interfaces up to and
including IRadio@1.2 are fully supported, although bits and pieces
of IRadio@1.4 are already there. Full support for IRadio@1.4 will
be added later as a separate task.
For now, only the standard (as in "defined by Google") IRadio
interfaces are used. Support for vendor-specific extensions is
planned and will be added later.
