wandboard: fix variable name so PXE boot works

All boards that support PXE booting use the pxefile_addr_r variable. Standardise
wandboard with this variable as pxe_addr_r isn't used anywhere else so it's a
typo.
Signed-off-by: NPeter Robinson <pbrobinson@gmail.com>
Acked-by: NFabio Estevam <fabio.estevam@nxp.com>

include/configs/wandboard.h

@@ -150,7 +150,7 @@
 		"if test $fdtfile = undefined; then " \
 			"echo WARNING: Could not determine dtb to use; fi; \0" \
 	"kernel_addr_r=" __stringify(CONFIG_LOADADDR) "\0" \
-	"pxe_addr_r=" __stringify(CONFIG_LOADADDR) "\0" \
+	"pxefile_addr_r=" __stringify(CONFIG_LOADADDR) "\0" \
 	"ramdisk_addr_r=0x13000000\0" \
 	"ramdiskaddr=0x13000000\0" \
 	"scriptaddr=" __stringify(CONFIG_LOADADDR) "\0" \