net: Convert CONFIG_TFTP_BLOCKSIZE to Kconfig

Convert CONFIG_TFTP_BLOCKSIZE to Kconfig, update defconfigs,
headers and whitelist.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/configs/colibri_imx6_defconfig b/configs/colibri_imx6_defconfig
index 712db45..a60d695 100644
--- a/configs/colibri_imx6_defconfig
+++ b/configs/colibri_imx6_defconfig
@@ -55,6 +55,7 @@
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
 CONFIG_IP_DEFRAG=y
+CONFIG_TFTP_BLOCKSIZE=16352
 CONFIG_DFU_MMC=y
 CONFIG_DM_GPIO=y
 CONFIG_DM_I2C=y