[Settings] Add per-netdevice settings block

Add a configuration settings block for each net device.  This will
provide the parent scope for settings applicable only to that network
device (e.g. non-volatile options stored on the NIC, options obtained via
DHCP, etc.).

Expose the MAC address as a setting.

src/include/gpxe/settings.h

@@ -17,7 +17,7 @@ struct in_addr;
 
 /** Settings block operations */
 struct settings_operations {
-	/** Set value of setting
+	/** Store value of setting
 	 *
 	 * @v settings		Settings block
 	 * @v tag		Setting tag number