[build] Expose build timestamp, build name, and product names

Expose the build timestamp (measured in seconds since the Epoch) and
the build name (e.g. "rtl8139.rom" or "ipxe.efi"), and provide the
product name and product short name in a single centralised location.

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

src/Makefile

@@ -96,6 +96,7 @@ SRCDIRS		+= config
 # automatic build system.
 #
 NON_AUTO_SRCS	:=
+NON_AUTO_SRCS	+= core/version.c
 NON_AUTO_SRCS	+= drivers/net/prism2.c
 
 # INCDIRS lists the include path