diff --git a/boot/zephyr/os.c b/boot/zephyr/os.c
index f913268..7c24eb3 100644
--- a/boot/zephyr/os.c
+++ b/boot/zephyr/os.c
@@ -22,9 +22,9 @@
 
 #include "os/os_heap.h"
 
-#ifdef MCUBOOT_USE_MBED_TLS
+#if !defined(CONFIG_MBEDTLS) && defined(MCUBOOT_USE_MBED_TLS)
 
-#define MBEDTLS_CONFIG_FILE CONFIG_MBEDTLS_CFG_FILE
+#define MBEDTLS_CONFIG_FILE MCUBOOT_MBEDTLS_CFG_FILE
 #include <mbedtls/platform.h>
 #include <mbedtls/memory_buffer_alloc.h>
 
diff --git a/boot/zephyr/prj-p256.conf b/boot/zephyr/prj-p256.conf
index c9cac66..71b52f3 100644
--- a/boot/zephyr/prj-p256.conf
+++ b/boot/zephyr/prj-p256.conf
@@ -4,9 +4,7 @@
 CONFIG_SYSTEM_CLOCK_DISABLE=y
 
 CONFIG_MAIN_STACK_SIZE=10240
-CONFIG_MBEDTLS=y
-CONFIG_MBEDTLS_BUILTIN=n
-CONFIG_MBEDTLS_CFG_FILE="config-asn1.h"
+MCUBOOT_MBEDTLS_CFG_FILE="config-asn1.h"
 # CONFIG_TINYCRYPT is not set
 # CONFIG_TINYCRYPT_ECC_DSA is not set
 # CONFIG_TINYCRYPT_SHA256 is not set