samples: zephyr: Use move swap for Zephyr tests

Change the configs for the Zephyr tests to use move swap, as this is the
code intended for future use.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/samples/zephyr/overlay-rsa.conf b/samples/zephyr/overlay-rsa.conf
index 539e779..c70349d 100644
--- a/samples/zephyr/overlay-rsa.conf
+++ b/samples/zephyr/overlay-rsa.conf
@@ -1,2 +1,3 @@
 # Kconfig overlay for building with RSA signatures
 CONFIG_BOOT_SIGNATURE_TYPE_RSA=y
+CONFIG_BOOT_SWAP_USING_MOVE=y