[Settings] Start revamping the configuration settings API.

Add the concept of an abstract configuration setting, comprising a (DHCP)
tag value and an associated byte sequence.

Add the concept of a settings namespace.

Add functions for extracting string, IPv4 address, and signed and
unsigned integer values from configuration settings (analogous to
dhcp_snprintf(), dhcp_ipv4_option(), etc.).

Update functions for parsing and formatting named/typed options to work
with new settings API.

Update NVO commands and config UI to use new settings API.

src/include/gpxe/settings_ui.h

@@ -7,8 +7,8 @@
  *
  */
 
-struct config_context;
+struct settings;
 
-extern int settings_ui ( struct config_context *context ) __nonnull;
+extern int settings_ui ( struct settings *settings ) __nonnull;
 
 #endif /* _GPXE_SETTINGS_UI_H */