mbedtls_x509write_crt_set_serial_raw() should take const serial buffer

[fwh-dc]

Suggested enhancement

int mbedtls_x509write_crt_set_serial_raw(mbedtls_x509write_cert *ctx,
unsigned char *serial, size_t serial_len)

should be
int mbedtls_x509write_crt_set_serial_raw(mbedtls_x509write_cert *ctx,
const unsigned char *serial, size_t serial_len)

Justification

I don't see any reason why mbedtls_x509write_crt_set_serial_raw() cannot take a const unsigned char *serial as argument. The rest of the api seems to take const pointers.