sim: Implement function in sim

The function `flash_area_id_from_image_slot` is used in the RAM_LOAD and
DIRECT_XIP configurations.  Define a version for use in the simulator.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/mcuboot-sys/csupport/run.c b/sim/mcuboot-sys/csupport/run.c
index 4786cbc..9302a59 100644
--- a/sim/mcuboot-sys/csupport/run.c
+++ b/sim/mcuboot-sys/csupport/run.c
@@ -438,6 +438,11 @@
     return fa->fa_device_id;
 }
 
+int flash_area_id_from_image_slot(int slot) {
+    /* For single image cases, just use the first image. */
+    return flash_area_id_from_multi_image_slot(0, slot);
+}
+
 void sim_assert(int x, const char *assertion, const char *file, unsigned int line, const char *function)
 {
     if (!(x)) {