update: tftp: dfu: Extend update_tftp() function to support DFU

This code allows using DFU defined mediums for storing data received via
TFTP protocol.

It reuses and preserves functionality of legacy code at common/update.c.

The update_tftp() function now accepts parameters - namely medium device
name and its number (e.g. mmc 1).

Without this information passed old behavior is preserved.

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