zephyr: cmake: Fix a typo in file CMakeLists.txt
zephyr: cmake: Fix a typo in file CMakeLists.txt

Replace "bur" with "but".

Signed-off-by: Ding Tao <07dff04f1a3ec2456623b40cc3a82a04949e5843@qq.com>

utzig/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,utzig/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,utzig/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,utzig/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,utzig/mcuboot