Use convenience function to checksum

That coreboot uses the IP checksum is an artifact, not a deliberate
requirement to be compatible with the Internet Protocole suite. Use
a wrapper to abstract the computation of coreboot's checksum.

Change-Id: I6491b9ba5efb9ffe5cb12a6172653a6ac80a1370
Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>
Reviewed-on: http://review.coreboot.org/497
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

payloads/libpayload/include/coreboot_tables.h

@@ -223,6 +223,11 @@ static inline u64 cb_unpack64(struct cbuint64 val)
 	return (((u64) val.hi) << 32) | val.lo;
 }
 
+static inline u16 cb_checksum(const void *ptr, unsigned len)
+{
+	return ipchksum(ptr, len);
+}
+
 /* Helpful macros */
 
 #define MEM_RANGE_COUNT(_rec) \