Convert CONFIG_TFTP_PORT to Kconfig

This converts the following to Kconfig:
   CONFIG_TFTP_PORT

Cc: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/snapper9g20_defconfig b/configs/snapper9g20_defconfig
index c6eca22..7eb512d 100644
--- a/configs/snapper9g20_defconfig
+++ b/configs/snapper9g20_defconfig
@@ -37,6 +37,7 @@
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_NET_RETRY_COUNT=20
+CONFIG_TFTP_PORT=y
 CONFIG_TFTP_TSIZE=y
 CONFIG_AT91_GPIO=y
 CONFIG_CMD_PCA953X=y