[romprefix] Add vendor branding facilities and guidelines

Some hardware vendors have been known to remove all gPXE-related
branding from ROMs that they build.  While this is not prohibited by
the GPL, it is a little impolite.

Add a facility for adding branding messages via two #defines
(PRODUCT_NAME and PRODUCT_SHORT_NAME) in config/general.h.  This
should accommodate all known OEM-mandated branding requirements.
Vendors with branding requirements that cannot be satisfied by using
PRODUCT_NAME and/or PRODUCT_SHORT_NAME should contact us so that we
can extended this facility as necessary.

src/config/general.h

@@ -9,6 +9,22 @@
 
 #include <config/defaults.h>
 
+/*
+ * Branding
+ *
+ * Vendors may use these strings to add their own branding to gPXE.
+ * PRODUCT_NAME is displayed prior to any gPXE branding in startup
+ * messages, and PRODUCT_SHORT_NAME is used where a brief product
+ * label is required (e.g. in BIOS boot selection menus).
+ *
+ * To minimise end-user confusion, it's probably a good idea to either
+ * make PRODUCT_SHORT_NAME a substring of PRODUCT_NAME or leave it as
+ * "gPXE".
+ *
+ */
+#define PRODUCT_NAME ""
+#define PRODUCT_SHORT_NAME "gPXE"
+
 /*
  * Timer configuration
  *