dfu: make data buffer size configurable

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 <[email protected]>
Cc: Pantelis Antoniou <[email protected]>
Cc: Tom Rini <[email protected]>
Cc: Lukasz Majewski <[email protected]>
Cc: Kyungmin Park <[email protected]>
Cc: Marek Vasut <[email protected]>
Cc: Wolfgang Denk <[email protected]>
Acked-by: Tom Rini <[email protected]>