[build] Allow product URI to be customised via config/branding.h

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

src/arch/i386/prefix/romprefix.S

 	.ascii	PRODUCT_NAME
 	.ascii	"\n"
 	.ascii	PRODUCT_SHORT_NAME
-	.asciz	" (http://ipxe.org)"
+	.ascii	" ("
+	.ascii	PRODUCT_URI
+	.asciz	")"
 	.size	init_message, . - init_message
 .ifeqs	BUSTYPE, "PCIR"
 init_message_pci:

src/config/branding.h

  */
 #define PRODUCT_NAME ""
 #define PRODUCT_SHORT_NAME "iPXE"
+#define PRODUCT_URI "http://ipxe.org"
 
 #include <config/local/branding.h>
 

src/usr/autoboot.c

 	 */
 	printf ( NORMAL "\n\n" PRODUCT_NAME "\n" BOLD PRODUCT_SHORT_NAME " %s"
 		 NORMAL " -- Open Source Network Boot Firmware -- "
-		 CYAN "http://ipxe.org" NORMAL "\n"
-		 "Features:", product_version );
+		 CYAN PRODUCT_URI NORMAL "\nFeatures:", product_version );
 	for_each_table_entry ( feature, FEATURES )
 		printf ( " %s", feature->name );
 	printf ( "\n" );