dell: new Dell plugin

For Dell-branded Novatel, Sierra and Ericsson modems.

The Novatel plugin will no longer accept every Dell-branded modem, which was
the current situation. Instead, a new Dell plugin will take care of probing for
the correct vendor string, and based on the results create a specific Novatel,
Sierra or Ericsson modem.

In order to properly support this, the Novatel, Sierra and MBM plugins now
export their implementations into non-inst libraries that the Dell plugin will
import.

Also, for now, the Dell plugin doesn't make any difference between e.g. Sierra
or Ericsson MBIM implementations, just a generic MBIM modem is created in both
cases, as that is anyway what the Ericsson MBM and Sierra plugins do already.

https://bugs.freedesktop.org/show_bug.cgi?id=86713

plugins/novatel/mm-plugin-novatel.c

@@ -77,9 +77,7 @@ G_MODULE_EXPORT MMPlugin *
 mm_plugin_create (void)
 {
     static const gchar *subsystems[] = { "tty", "net", "usb", NULL };
-    static const guint16 vendors[] = { 0x1410, /* Novatel */
-                                       0x413c, /* Dell */
-                                       0 };
+    static const guint16 vendors[] = { 0x1410, 0 };
     static const mm_uint16_pair forbidden_products[] = { { 0x1410, 0x9010 }, /* Novatel E362 */
                                                          { 0, 0 } };
     static const MMAsyncMethod custom_init = {