zephyr: Added hooks for example

Example (very basic) hook implementation.
Added case description for test-build with these hooks
and multi-image feature.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/boot/zephyr/sample.yaml b/boot/zephyr/sample.yaml
index 35b4f1e..4f8ee11 100644
--- a/boot/zephyr/sample.yaml
+++ b/boot/zephyr/sample.yaml
@@ -40,3 +40,10 @@
     integration_platforms:
       - nrf52840dk_nrf52840
     tags: bootloader_mcuboot
+  sample.bootloader.mcuboot.hooks_multi:
+    extra_args: DTC_OVERLAY_FILE=./boards/nrf52840dk_ram_multi.overlay
+      OVERLAY_CONFIG=./boards/nrf52840dk_hooks_sample_overlay.conf
+    platform_allow: nrf52840dk_nrf52840
+    integration_platforms:
+      - nrf52840dk_nrf52840
+    tags: bootloader_mcuboot