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_t30_defconfig b/configs/colibri_t30_defconfig
index 3d25f10..769f6f3 100644
--- a/configs/colibri_t30_defconfig
+++ b/configs/colibri_t30_defconfig
@@ -28,6 +28,7 @@
 CONFIG_OF_LIVE=y
 CONFIG_DEFAULT_DEVICE_TREE="tegra30-colibri"
 CONFIG_IP_DEFRAG=y
+CONFIG_TFTP_BLOCKSIZE=16352
 CONFIG_SPL_DM=y
 CONFIG_DFU_MMC=y
 CONFIG_DFU_RAM=y