SysFs interface. I can't export gpio pins in a Xilinx's Board (Zybo and other)

Using a linux-kernel compiled as it is described here, I'm trying to make a led blinking following this wiki: Linux GPIO Driver. I'm working with a Zybo-board of Xilinx.

I enabled the kernel options:

CONFIG_GPIO_SYSFS=y
CONFIG_SYSFS=y
CONFIG_GPIO_XILINX=y

I checked that I have mounted in /sys the SysFs

I want to configure the pin 7 of the MIO port because it is attached to the led LD4 in the board. So I used this expression:

echo 7 > /sys/class/gpio/export

And I always obtain this error:

export_store: invalid GPIO 7
ash: write error: Invalid argument

I have also tried to export the values 145 (138+7) because of the explanation I found here: forum_link and 86 because this guy got the things working basic_example. But I always obtained the same error.

Could you give me some help? Maybe a I have to use other offset? Or is it more related with permission configuration??