dfu: tftp: update: Add dfu_write_from_mem_addr() function

dfu: tftp: update: Add dfu_write_from_mem_addr() function

This function allows writing via DFU data stored from fixed buffer address
(like e.g. loadaddr env variable).

Such predefined buffers are used in the update_tftp() code. In fact this
function is a wrapper on the dfu_write() and dfu_flush().

Signed-off-by: Lukasz Majewski <[email protected]>
Acked-by: Joe Hershberger <[email protected]>