shell: increase stack size for OpenThread
shell: increase stack size for OpenThread

Default shell stack size is not always enough when used with
OpenThread. Increasing it to the found adequate value.

Signed-off-by: Eduardo Montoya <c6d924925ebf82d772472f7877e771cedf439387@nordicsemi.no>

galak/zephyr,nashif/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr