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/xilinx_versal_virt_defconfig b/configs/xilinx_versal_virt_defconfig
index 9000573..3ce6c03 100644
--- a/configs/xilinx_versal_virt_defconfig
+++ b/configs/xilinx_versal_virt_defconfig
@@ -43,6 +43,7 @@
 CONFIG_OF_BOARD=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_IP_DEFRAG=y
+CONFIG_TFTP_BLOCKSIZE=4096
 CONFIG_SPL_DM_SEQ_ALIAS=y
 CONFIG_DM_GPIO=y
 CONFIG_DM_I2C=y