Change usage of AT_CellularContext::stack_type_supported to
AT_CellularBase::get_property. This way we can rid of
targets overriding stack_type_supported and delete
unnecessary classes and simplify new targets.
Add override for the virtual method get_connection_status() inherited
from NetworkInterface. The method in the base class returns
NSAPI_STATUS_ERROR_UNSUPPORTED. The CellularContext has the member
_connection_status, which means that we could return this.