dfu: make data buffer size configurable

Dfu transfer uses a buffer before writing data to the
raw storage device. Make the size (in bytes) of this buffer
configurable through environment variable "dfu_bufsiz".
Defaut value is configurable through CONFIG_SYS_DFU_DATA_BUF_SIZE

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Tom Rini <trini@ti.com>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Acked-by: Tom Rini <trini@ti.com>
diff --git a/README b/README
index 5c343da..85e5a8d 100644
--- a/README
+++ b/README
@@ -1392,6 +1392,12 @@
 		CONFIG_DFU_NAND
 		This enables support for exposing NAND devices via DFU.
 
+		CONFIG_SYS_DFU_DATA_BUF_SIZE
+		Dfu transfer uses a buffer before writing data to the
+		raw storage device. Make the size (in bytes) of this buffer
+		configurable. The size of this buffer is also configurable
+		through the "dfu_bufsiz" environment variable.
+
 		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