[build] Allow setting help text URI to be customised via config/branding.h

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

src/config/branding.h

  */
 #define PRODUCT_COMMAND_URI "http://ipxe.org/cmd/%s"
 
+/*
+ * Setting help messages
+ *
+ * iPXE setting help messages include a URI constructed from the
+ * setting name, such as
+ *
+ *   "http://ipxe.org/cfg/initiator-iqn"
+ *
+ * The iPXE web site includes documentation for the settings used by
+ * iPXE, including:
+ *
+ * - details of the corresponding DHCP option number.
+ *
+ * - details of the corresponding ISC dhcpd option name.
+ *
+ * - examples of using the setting from the iPXE command line, or in
+ *   iPXE scripts.
+ *
+ * - examples of configuring the setting via a DHCP server.
+ *
+ * - a formal description of the setting.
+ *
+ * - links to documentation for related settings.
+ *
+ * - links to documentation for relevant build options.
+ *
+ * - general notes about the setting.
+ *
+ * If you want to provide your own documentation for all of the
+ * settings used by iPXE, rather than using the existing support
+ * infrastructure provided by http://ipxe.org, then you may define a
+ * custom URI to be included within setting help messages.
+ *
+ * Note that the custom URI is a printf() format string which must
+ * include a format specifier for the setting name.
+ *
+ * [ Please also note that the existing documentation is licensed
+ *   under Creative Commons terms which require attribution to the
+ *   iPXE project and prohibit the alteration or removal of any
+ *   references to "iPXE". ]
+ */
+#define PRODUCT_SETTING_URI "http://ipxe.org/cfg/%s"
+
 #include <config/local/branding.h>
 
 #endif /* CONFIG_BRANDING_H */

src/hci/tui/settings_ui.c

 	msg ( INFO_ROW, "%s - %s", buf, widget->row.setting.description );
 	attroff ( A_BOLD );
 	color_set ( CPAIR_URL, NULL );
-	msg ( ( INFO_ROW + 1 ), "http://ipxe.org/cfg/%s",
-	      widget->row.setting.name );
+	msg ( ( INFO_ROW + 1 ), PRODUCT_SETTING_URI, widget->row.setting.name );
 	color_set ( CPAIR_NORMAL, NULL );
 }
 

src/interface/efi/efi_snp_hii.c

 #include <ipxe/efi/efi_hii.h>
 #include <ipxe/efi/efi_snp.h>
 #include <ipxe/efi/efi_strings.h>
+#include <config/branding.h>
 
 /** EFI platform setup formset GUID */
 static EFI_GUID efi_hii_platform_setup_formset_guid
 		previous = setting;
 		name_id = efi_ifr_string ( ifr, "%s", setting->name );
 		prompt_id = efi_ifr_string ( ifr, "%s", setting->description );
-		help_id = efi_ifr_string ( ifr, "http://ipxe.org/cfg/%s",
+		help_id = efi_ifr_string ( ifr, PRODUCT_SETTING_URI,
 					   setting->name );
 		question_id = setting->tag;
 		efi_ifr_string_op ( ifr, prompt_id, help_id,