boot: zephyr: add ECIES-P256 configuration

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/boot/zephyr/prj.conf b/boot/zephyr/prj.conf
index d40307a..89e55f5 100644
--- a/boot/zephyr/prj.conf
+++ b/boot/zephyr/prj.conf
@@ -7,6 +7,7 @@
 CONFIG_MBEDTLS_CFG_FILE="mcuboot-mbedtls-cfg.h"
 
 CONFIG_BOOT_ENCRYPT_RSA=n
+CONFIG_BOOT_ENCRYPT_EC256=n
 
 CONFIG_BOOT_BOOTSTRAP=n