zephyr: Use a smaller sha256 implementation

Reduce the size of the sha256 implementation by 1300 bytes by using a
slower implementation.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
diff --git a/boot/zephyr/include/config-ed25519.h b/boot/zephyr/include/config-ed25519.h
index 7e43708..95b299e 100644
--- a/boot/zephyr/include/config-ed25519.h
+++ b/boot/zephyr/include/config-ed25519.h
@@ -60,6 +60,7 @@
 #define MBEDTLS_MD_C
 #define MBEDTLS_OID_C
 #define MBEDTLS_SHA256_C
+#define MBEDTLS_SHA256_SMALLER
 #define MBEDTLS_SHA224_C
 #define MBEDTLS_SHA512_C
 #define MBEDTLS_AES_C