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/common/cmd_pxe.c b/common/cmd_pxe.c
index 9eac5c6..5cde5b6 100644
--- a/common/cmd_pxe.c
+++ b/common/cmd_pxe.c
@@ -1568,7 +1568,7 @@
 
 	destroy_pxe_menu(cfg);
 
-	copy_filename(BootFile, "", sizeof(BootFile));
+	copy_filename(net_boot_file_name, "", sizeof(net_boot_file_name));
 
 	return 0;
 }