net: Add a separate file for IP checksumming

Move the checksum code out into its own file so it can be used elsewhere.
Also use a new version which supports a length which is not a multiple of
2 and add a new function to add two checksums.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/net/Makefile b/net/Makefile
index 9425950..e9cc8ad 100644
--- a/net/Makefile
+++ b/net/Makefile
@@ -7,6 +7,7 @@
 
 #ccflags-y += -DDEBUG
 
+obj-y += checksum.o
 obj-$(CONFIG_CMD_NET)  += arp.o
 obj-$(CONFIG_CMD_NET)  += bootp.o
 obj-$(CONFIG_CMD_CDP)  += cdp.o