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 <l.majewski@majess.pl>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>