[settings] Clarify usage of the term "named setting"

There are currently two conflicting usages of the term "named setting"
within iPXE: one refers to predefined settings (such as show up in the
"config" UI), the other refers to settings identified by a name (such
as "net0.dhcp/ip").

Split these usages into the term "predefined setting" and "named
setting" to avoid ambiguity.

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

src/net/netdev_settings.c

@@ -35,7 +35,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
  *
  */
 
-/** Network device named settings */
+/** Network device predefined settings */
 struct setting mac_setting __setting ( SETTING_NETDEV ) = {
 	.name = "mac",
 	.description = "MAC address",