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>
diff --git a/drivers/dfu/Kconfig b/drivers/dfu/Kconfig
index 0eec00b..10196f3 100644
--- a/drivers/dfu/Kconfig
+++ b/drivers/dfu/Kconfig
@@ -14,8 +14,13 @@
 	depends on NET
 
 if DFU
+config DFU_WRITE_ALT
+	bool
+	default n
+
 config DFU_TFTP
 	bool "DFU via TFTP"
+	select DFU_WRITE_ALT
 	select DFU_OVER_TFTP
 	help
 	  This option allows performing update of DFU-managed medium with data