core: mbedtls: configure bignum word size

Configures mbedtls bignum word size (mbedtls_mpi_uint and mbedtls_mpi_sint)
as 32-bit or 64-bit depending on arm architecture.

Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/lib/libmbedtls/include/mbedtls_config_kernel.h b/lib/libmbedtls/include/mbedtls_config_kernel.h
index fd8b223..5533049 100644
--- a/lib/libmbedtls/include/mbedtls_config_kernel.h
+++ b/lib/libmbedtls/include/mbedtls_config_kernel.h
@@ -4,6 +4,12 @@
 #define __MBEDTLS_CONFIG_KERNEL_H
 
 #ifdef CFG_CORE_MBEDTLS_MPI
+#ifdef ARM32
+#define MBEDTLS_HAVE_INT32
+#endif
+#ifdef ARM64
+#define MBEDTLS_HAVE_INT64
+#endif
 #define MBEDTLS_BIGNUM_C
 #define MBEDTLS_GENPRIME
 #endif