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/net/Kconfig b/net/Kconfig
index 1a6fda9..68cecf7 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -29,4 +29,10 @@
 	  Selecting this will enable IP datagram reassembly according
 	  to the algorithm in RFC815.
 
+config TFTP_BLOCKSIZE
+	int "TFTP block size"
+	default 512
+	help
+	  Default TFTP block size.
+
 endif   # if NET