ARMv7l: fix MAX_TFTP_PATH_LEN size for nixos compatibility

hey,

i've been using the master branch of u-boot on the odroid xu4 now! i had to write this patch in order to make it work with NIXOS.

would be great if you could apply it. please review it, not 100% sure if it is correct.

best wishes,
joachim

MAX_TFTP_PATH_LEN-increase.patch

A patch to make NixOS on the Odroid XU4 booting with U-Boot/extlinux
possible. If not applied U-Boot can't process the long string:
FDTDIR
../nixos/ccn1vfjww519c56wy8jkc0p6acw4asf3-linux-4.2.0-5eb40d45b291ee129cbfac2073f1ca7aa32ff4c5-dtbs/exynos5422-odroidxu4.dtb
---

index 080b376..67dd647 100644
@@ -19,7 +19,7 @@
 #include "menu.h"
 #include "cli.h"
 
-#define MAX_TFTP_PATH_LEN 127
+#define MAX_TFTP_PATH_LEN 512
 
 const char *pxe_default_paths[] = {
 #ifdef CONFIG_SYS_SOC