dfu: rename dfu_tftp_write() to dfu_write_by_name()

This function is essentially independent from tftp, and will also be
utilised in implementing UEFI capsule update in a later commit.
So just give it a more generic name.
In addition, a new configuration option, CONFIG_DFU_WRITE_ALT, was
introduced so that the file will be compiled with different options,
particularly one added in a later commit.

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