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_t20_defconfig b/configs/colibri_t20_defconfig
index 8247248..d6a20ca 100644
--- a/configs/colibri_t20_defconfig
+++ b/configs/colibri_t20_defconfig
@@ -37,6 +37,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="tegra20-colibri"
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_IP_DEFRAG=y
+CONFIG_TFTP_BLOCKSIZE=1536
 CONFIG_SPL_DM=y
 CONFIG_DFU_MMC=y
 CONFIG_DFU_RAM=y