boot/zephyr: Added single-slot Zephyr-RTOS test build

Added configuration for build in Zephyr-RTOS CI MCUBoot
in CONFIG_SINGLE_APPLICATION_SLOT=y mode.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/boot/zephyr/single_slot.conf b/boot/zephyr/single_slot.conf
new file mode 100644
index 0000000..34cc92a
--- /dev/null
+++ b/boot/zephyr/single_slot.conf
@@ -0,0 +1 @@
+CONFIG_SINGLE_APPLICATION_SLOT=y