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

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

src/config/branding.h

 #define PRODUCT_SHORT_NAME "iPXE"
 #define PRODUCT_URI "http://ipxe.org"
 
+/*
+ * Tag line
+ *
+ * If your PRODUCT_SHORT_NAME is longer than the four characters used
+ * by "iPXE", then the standard tag line "Open Source Network Boot
+ * Firmware" is unlikely to fit neatly onto the screen.
+ */
+#define PRODUCT_TAG_LINE "Open Source Network Boot Firmware"
+
 /*
  * Error messages
  *

src/usr/autoboot.c

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