zephyr-openthread-rcp: use OpenThread version 1.2

We're now using OT version 1.2 so set that option in the zephyr build.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@huawei.com>

meta-zephyr-core/recipes-kernel/zephyr-kernel/zephyr-openthread-rcp.bb

@@ -2,7 +2,7 @@ include zephyr-sample.inc
 
 ZEPHYR_SRC_DIR = "${S}/samples/net/openthread/coprocessor"
 
-EXTRA_OECMAKE += "-DCONF_FILE="prj.conf overlay-rcp.conf overlay-usb-nrf-br.conf" -DDTC_OVERLAY_FILE="usb.overlay""
+EXTRA_OECMAKE += "-DCONF_FILE="prj.conf overlay-rcp.conf overlay-usb-nrf-br.conf" -DDTC_OVERLAY_FILE="usb.overlay" -DCONFIG_OPENTHREAD_THREAD_VERSION_1_2=y"
 
 # The overlay config and OpenThread itself imposes some specific requirements
 # towards the boards (e.g. flash layout and ieee802154 radio) so we need to