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.