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.h

@@ -134,4 +134,8 @@ elf_headers(const struct buffer *pinput,
 	    Elf64_Phdr **pphdr,
 	    Elf64_Shdr **pshdr);
 
+/* cbfs-mkpayload.c */
+void xdr_segs(struct buffer *output,
+	      struct cbfs_payload_segment *segs, int nseg);
+
 #endif