Change the linux payload generator to use the standard header generator

When I changed mkpayload, I did not realize we had a duplicate
block of code in the linux payload code. Have it use the same
header generator as the standard payload code does.

Change-Id: Ie39540089ce89b704290c89127da4c7b051ecb0e
Signed-off-by: Ronald G. Minnich <rminnich@google.com>
Reviewed-on: http://review.coreboot.org/5115
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

util/cbfstool/cbfs-mkpayload.c

@@ -32,7 +32,7 @@
 /* serialize the seg array into the buffer.
  * The buffer is assumed to be large enough.
  */
-static void xdr_segs(struct buffer *output,
+void xdr_segs(struct buffer *output,
 	struct cbfs_payload_segment *segs, int nseg)
 {
 	struct buffer outheader;