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>