dfu: Support larger than memory transfers.

Previously we didn't support upload/download larger than available
memory.  This is pretty bad when you have to update your root filesystem
for example.

This patch removes that limitation (and the crashes when you transfered
any file larger than 4MB) by making raw image writes be done in chunks
and making file maximum size be configurable.

The sequence number is a 16 bit counter; make sure we handle rollover
correctly. This fixes the wrong transfers for large (> 256MB) images.

Also utilize a variable to handle initialization, so that we don't rely
on just the counter sent by the host.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>
Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/README b/README
index f7f41bc..7879eef 100644
--- a/README
+++ b/README
@@ -1357,6 +1357,13 @@
 		CONFIG_DFU_MMC
 		This enables support for exposing (e)MMC devices via DFU.
 
+		CONFIG_SYS_DFU_MAX_FILE_SIZE
+		When updating files rather than the raw storage device,
+		we use a static buffer to copy the file into and then write
+		the buffer once we've been given the whole file.  Define
+		this to the maximum filesize (in bytes) for the buffer.
+		Default is 4 MiB if undefined.
+
 - Journaling Flash filesystem support:
 		CONFIG_JFFS2_NAND, CONFIG_JFFS2_NAND_OFF, CONFIG_JFFS2_NAND_SIZE,
 		CONFIG_JFFS2_NAND_DEV