[linux] Give tap devices a name and bus type

Give tap devices a meaningful name, and avoid segmentation faults when
attempting to retrieve ${net0/bustype} by assigning a new bus type for
tap devices.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/device.h

@@ -54,6 +54,9 @@ struct device_description {
 /** ISA bus type */
 #define BUS_TYPE_ISA 5
 
+/** TAP bus type */
+#define BUS_TYPE_TAP 6
+
 /** A hardware device */
 struct device {
 	/** Name */