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/apalis-imx8qm_defconfig b/configs/apalis-imx8qm_defconfig
index 7848b68..584e54d 100644
--- a/configs/apalis-imx8qm_defconfig
+++ b/configs/apalis-imx8qm_defconfig
@@ -26,6 +26,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="fsl-imx8qm-apalis"
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_IP_DEFRAG=y
+CONFIG_TFTP_BLOCKSIZE=4096
 CONFIG_CLK_IMX8=y
 CONFIG_CPU=y
 CONFIG_DM_GPIO=y