Enable MBEDTLS_FS_IO whenever possible

We enable MBEDTLS_FS_IO whenever MBED_CONF_FILESYSTEM_PRESENT is
defined in mbed OS. It is assumed that MBED_CONF_FILESYSTEM_PRESENT
defines at least the filesystem functionality required by MBEDTLS_FS_IO

Author: Andres Amaya Garcia

features/mbedtls/platform/inc/platform_mbed.h

@@ -21,6 +21,10 @@
 #define MBEDTLS_ENTROPY_HARDWARE_ALT
 #endif
 
+#if defined(MBED_CONF_FILESYSTEM_PRESENT)
+#define MBEDTLS_FS_IO
+#endif
+
 #if defined(MBEDTLS_CONFIG_HW_SUPPORT)
 #include "mbedtls_device.h"
 #endif