image encryption: fix enc_state array indexing for zephyr

enc_state table was indexed with assumption that
image flash area are subsequent and increasing numbers.
It might not be true while building zephyr.

Patch introduce flash_area_id_to_image_slot() implementation for
the zephyr port and uses it to assign proper slot number.
This API is already available in MyNewt.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/sim/mcuboot-sys/csupport/run.c b/sim/mcuboot-sys/csupport/run.c
index eb5b155..fdee14c 100644
--- a/sim/mcuboot-sys/csupport/run.c
+++ b/sim/mcuboot-sys/csupport/run.c
@@ -384,6 +384,19 @@
     return 0;
 }
 
+int flash_area_id_to_image_slot(int area_id)
+{
+    switch (area_id) {
+    case FLASH_AREA_IMAGE_PRIMARY:
+        return 0;
+    case FLASH_AREA_IMAGE_SECONDARY:
+        return 1;
+    default:
+        printf("Unsupported image area ID\n");
+        abort();
+    }
+}
+
 void sim_assert(int x, const char *assertion, const char *file, unsigned int line, const char *function)
 {
     if (!(x)) {