[U-Boot] dfu: make data buffer size configurable

[Wolfgang Denk]

Dear Heiko Schocher,

In message <51B555D7.5010001 at denx.de> you wrote:
> 
> Ok, I can change this. Envvar name "dfu_data_buf_size" is ok?

Such long names are a paint to type. As we can't buffer anything else
but data, we should be able to omit this, i. e. what about

	dfu_bufsiz

["bufsiz" as used for example as BUFSIZ in C89 stdio.h]

Best regards,

Wolfgang Denk

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