net: cosmetic: Fixup var names related to boot file

The variables around the bootfile were inconsistent and used CamelCase.
Update them to make the code more readable.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/net/eth.c b/net/eth.c
index c1d6b04..f06fdb2 100644
--- a/net/eth.c
+++ b/net/eth.c
@@ -66,7 +66,8 @@
 
 	s = getenv("bootfile");
 	if (s != NULL)
-		copy_filename(BootFile, s, sizeof(BootFile));
+		copy_filename(net_boot_file_name, s,
+			      sizeof(net_boot_file_name));
 }
 
 static int eth_mac_skip(int index)