Add Zephyr support for encrypted images

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/zephyr/prj.conf b/boot/zephyr/prj.conf
index aec69e8..11e778a 100644
--- a/boot/zephyr/prj.conf
+++ b/boot/zephyr/prj.conf
@@ -7,6 +7,8 @@
 CONFIG_MAIN_STACK_SIZE=10240
 CONFIG_MBEDTLS_CFG_FILE="mcuboot-mbedtls-cfg.h"
 
+CONFIG_BOOT_ENCRYPT_RSA=n
+
 ### Default to RSA
 CONFIG_BOOT_SIGNATURE_TYPE_RSA=y
 CONFIG_BOOT_SIGNATURE_TYPE_ECDSA_P256=n