dfu: tftp: Kconfig: Add Kconfig entry for dfu tftp feature

The dfu tftp feature can be now enabled via Kconfig. This
commit provides necessary code for it.

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

drivers/dfu/Kconfig

@@ -0,0 +1,10 @@
+menu "DFU support"
+
+config DFU_TFTP
+	bool "DFU via TFTP"
+	help
+	  This option allows performing update of DFU managed medium with data
+	  send via TFTP boot.
+	  Detailed description of this feature can be found at ./doc/README.dfutftp
+
+endmenu