Boot: Add CTR mode support to mbedcrypto config

Used by the encrypted image support in MCUboot.

Change-Id: Icc77eb18d81e418c0f02e8824a640498f699d692
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/bl2/ext/mcuboot/include/mcuboot-mbedtls-cfg.h b/bl2/ext/mcuboot/include/mcuboot-mbedtls-cfg.h
index 7e52d34..010a4f8 100644
--- a/bl2/ext/mcuboot/include/mcuboot-mbedtls-cfg.h
+++ b/bl2/ext/mcuboot/include/mcuboot-mbedtls-cfg.h
@@ -57,6 +57,7 @@
 #define MBEDTLS_OID_C
 #define MBEDTLS_SHA256_C
 #define MBEDTLS_AES_C
+#define MBEDTLS_CIPHER_MODE_CTR
 
 /* Save RAM by adjusting to our exact needs */
 #if MCUBOOT_SIGN_RSA_LEN == 3072