zephyr: use REQUIRED in dt_nodelabel calls to fail early and with meaningful message

When the devicetree was missing a label, I'd get this kind of errors,
which were not very helpful:
CMake Error at workspace/zephyr/cmake/modules/extensions.cmake:3979 (message):
  dt_prop(erase_size_slot1 ...) missing required argument: PATH
Call Stack (most recent call first):
  CMakeLists.txt:496 (dt_prop)

The new error message looks like this:
CMake Error at workspace/zephyr/cmake/modules/extensions.cmake:3748 (message):
  required nodelabel not found: slot1_partition

Signed-off-by: Benedek Kupper <kupper.benedek@gmail.com>