Add MBEDTLS_FS_IO guard on default NV seed defines in platform.h
Add MBEDTLS_FS_IO guard on default NV seed defines in platform.h

The default implementation won't work without MBEDTLS_FS_IO, so
leave undefined otherwise.

NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls