dfu: add dfu_write_by_alt()

This function is a variant of dfu_write_by_name() and takes a DFU alt
setting number for dfu configuration.

It will be utilised to implement UEFI capsule management protocol for
raw image in a later commit.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>